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Fuel Filters, Gas Caps, Petcocks, Tanks & Seats,
Fuel Hose, Gas Flow Amounts; choke lever marking on clam-shell models;ETC.


Copyright, 2014, R. Fleischer

Additional information is in article 12A; this is a link!


PETCOCKS in general:

Petcocks are fastened to the tank by right and left hand threads, both on each petcock, versus the tank. 
That is, as you rotate the large "nut" CW (facing from below), that tightens the nut to the tank while at the same time it brings up the petcock.
The Petcock large nut has to be engaged the proper amount onto the petcock, BEFORE screwing it upwards to begin to engage the proper amount of tank threads.  You will have to fiddle with this a few times, until you get the right amount of petcock and nut threads assembled, to start the tank fastening process.

BMW has used numerous petcocks, but there are FIVE TYPES of petcocks normally seen for our airheads.  All except the /5 bullet types are rather easily re-buildable. Usually this means that you unscrew some knurled or slot "nut" at the handle area (after removing a black trim cup, if it is there on your version), and you can replace a gasket that might be bad, or clean and lightly silicon grease the moving parts. You then have a much smoother operating petcock.  On some there are one or two dimpled discs, which can be repaired with a shaped punch, lightly hit, for a better detent action.  These discs have a locating tab.  They confuse some folks, so take notice of how assembled as you take things apart.  You CAN figure it out, if you forgot to take notice.

There is a version with a gasket you need to make, I will get into all these versions in this article, more or less anyway.

MOST of all the petcock's except the early /5 bullet type are similar in INternal construction; some variances, but the basics are the same, perforated discs.   Early /5 were different, the rotating handle effecting gas flow from one or another port.

NOTE:   Some have been able to force-twist the /5 Everbest petcocks apart for repairs.
Here is a link to an article:

NOTE:  Here is a link to a parts kit:

NOTE:  Here is a link to a guy with an Ebay store.  He can, I think, service yours; or, has cork gaskets, ETC:

The early bullet-style petcocks popular on the /5, which look like there is no way to take them apart, are sometimes re-buildable. Frankly, while I would not bother with the early bullet styles, for purists, and originality purposes, it may be worth a try, and you can take a look at Vech's website:  Where he has some decent information on how to rebuild one.  The problem is finding the article.  Vetch's website is generally not set-up for internal hyperlinks that are easy to get to.

However, this MAY work:

Here is a link to Bing Agency's petcock page, useful for the photos, etc.  BMW dealership prices may be LESS.

Here is a sketch of the original slash-5 bullet petcock:

NOTE!   On these /5 bikes with the above petcock, the CHOKE cable tends to rub
the underside of the bullet area.  That cable can, and has, been known to wear through the
  petcock!  I suggest you add a piece of rubber hose over the cable where it might rub the petcock.

These early /5 petcocks, on the engine side of the handle, look something like a very large tapered round bullet.  They are snapped together, and it is difficult to take them apart without breaking them. This CAN be done, SOMEtimes.   They have an easily removable nipple at the bottom, inside of which is a small fuel screen.   Because they are the Classic /5 petcocks, you MIGHT want to TRY to overhaul one. 
Repairing one is a bit time consuming, but you can retain the classic look of your /5.  

You have nothing much to lose, as you can always substitute a later petcock, so here is a way to go about it:
The early /5-style of petcock is press-assembled, and staked.  To disassemble it, you must trim away the body slightly, of the crimp area.    Do NOT remove excessive material in that trimming.   Then put the HANDLE of the petcock in a soft jawed vise, and pull and twist the body, and it will come apart.   You will need some 1/8" thick cork to finish the overhaul job.   Most auto-parts stores have some.  You could use a piece of gasket material, etc.  You will have to cut the cork to fit, and then SILICONE grease it, roll it sort of cylindrical, insert it into the body...seam towards side...install squarely.   Grease the removed plug part and install will take considerable pressure to do this.  Now you need to crimp the area so it cannot come apart on its own.   After that is done, turn the handle (fair amount of force probably needed) to ON position.  Drill down the main tube through the cork, and the outlet too.  Blow out the cork dust.  Repeat for the reserve position tube and outlet.  Clean out, and reinstall.

NOTE:  The original "straight" type petcock used early-on after the bullet petcock is no longer available from BMW dealerships.
Check out this website page for availability of that type: Series 2 valve Twin


ALL the petcock styles except the bullet nose /5 type are fairly easy to take apart, clean, and repair...typically this is done because they have begun to either leak or are stiff operating.  

There are two types of GERMA petcocks, and a couple of other types too. Some versions have in-tank screens.  For the IN-TANK tall screen type, a separate sealing washer is used at the screen bottom where it fits into the top of the petcock and it is best to replace that old screen with the later bonded-metal-washer style screen that BMW offers.  NO separate cambric washer is then used.   The reason you want to replace the early style screen is that it CAN let debris through where the screen meets the petcock base...., and re-forming the roundness of the screen where it fits into the petcock was also typically necessary whenever the screen was removed.    Stated differently, y
ou had to 'form' the lower end of the screen into a proper round shape, then insert the screen a bit into the cambric washer, then assemble, making sure the screen still fit into the recess in the petcock top area, and then attach to the tank. It was a bit of a hassle to do this perfectly.    That type petcock often had a spacer, usually white, near the top, INside of the screen, not a critical item if missing. The problem with those early screens is that folks simply did not pay attention to assembling the screen, washer, and petcock upper cavity, and the screen then did little 'screening' at the bottom, letting junk get into carburetor. BMW changed the design of the in-tank screen, and if you purchase an in-tank screen, you will now, or should!...get one with the washer being metal and permanently bonded to the screen. NO special sealing washer is needed, that is, you do NOT reuse the old washer, you do NOT USE IT or any, at all.  

Have clean surfaces and a good grunt on that outside nut....with equal threads being engaged.  This applies to all petcocks when installing them.

Most other petcocks have the screen IN the outlet, remove the outlet part to see and clean it.  These petcocks have the outlet straight down, and have a separate smaller nut at that outlet, so as to remove the outlet stub and get to that filter. That does NOT mean that all straight down outlets have screens. Easy to find out, simply remove the hose, and unscrew the lower nut, see if a filter there...or a place for one, the previous owner could have left it out.     The outlet, when the removable type, is called a tap spigot, or??, and they are available, or were, in both metal and plastic.  Guess which one does not like to be bent sideways when removing a hose?  Any spigot may have a groove in the part that fits up to the petcock, hidden by the nut...that groove is where the filter screen fits, some types of filter screens had to be hand-formed to fit just right, others had stiff end structures.

I cannot emphasize more that you should ADD an aftermarket filter between any petcock and the carburetor.  More later in this article.

TWO types of petcocks, and these are generally the last petcock versions, do not have an outlet screen filter, these have non-removable horizontal fuel outlets so there are NO nuts on the outlets, the outlet spigot is part of the casting.   One of these will look similar to the late version but the threaded ring, which is still cross-slotted, has NO outer edge knurling, HAS a radial slot and has the internal operating 'gasket', with the ports to allow the fuel flow, made of CORK.   Change that to rubber part #16-12-1-235-551.  The cork can swell and make the petcock stiff to operate, and it can also shrink, and cause leaking.  GENERALLY that cork to rubber change is for the USA and Canada shipped models R60/6 through R100S.  There are sub-variations of these petcocks, in how the outer ring appears, tightened for pressure, or not, etc. 
The last version of these had outer edge knurling and had the rubber part AND a rather stiff coil spring, and seals better.

***These horizontal outlet late style petcocks are HANDED, that is, there is a left hand side and a right hand side.  This information is hardly noted anyplace but here by me.  When installing to the fuel tank, if you install these petcocks so the OUTLET is pointing REARWARDS, then the curve of the fuel hose will be much nicer, shorter, and the hose might last longer.  There are a FEW models that will need the outlet pointed forward, due to tank and other variations such as smog equipment.

The working face of the inner flat side of the handle, must be free of deep scratches.  

Use faint amounts of silicone grease on petcock parts, the petcock will last longer, work smoother.

Generally speaking, whilst the various inlet and/or outlet screens should be cleaned with your yearly tank cleaning, there is no need to disassemble the main petcock internals until action is overly-stiff.   THEN, except for that cork mentioned above to be changed to rubber, there is usually nothing to do but clean, and apply silicone grease very thinly, and reassemble.  Obviously you do not want so much grease that you plug the disc holes.

Except for the early /5  'bullet' style petcock, the other types of petcocks are easily taken apart, but sometimes, on the LATER, non-Germa ones,  one needs a good grunt to push the threaded 'nut' back into the body to get its threads started properly. Some folks think this is easier by using a bench vise (carefully, don't over-squeeze!) on the NUT portion, and pushing the petcock portion, squarely...SQUARELY!! the nut. NUT here means the knurled or slot part (possibly under a black plastic cover, which pries off).

For the petcocks with or without a black plastic cover around the lever area, this will probably apply:

Slowly remove the black cover. Pry evenly, all around, a bit here, a bit there. You can use a small piece of wood or plastic, or? to avoid marring.  You might want to put the whole petcock in moderately hot water, and use a plastic tool.  The water will soften the black plastic cover, expand it a bit, and make it easier to remove....and the plastic tool is less likely to gouge or mar the material.  There have been instances of loose caps, where someone has glued it to the petcock, just be aware of this, as it may take more work to get the cap off, NEEDED to remove the handle and innards.

Underneath the plastic cap you will probably find a sort of knurled edge that must be gripped in order to separate the metal cap plug from the body.  I made a special tool for this about 30 years ago, can't find the darn thing right now, but you CAN grip the edge in a vice, and then rotate the rest of the petcock around it. It is better if you had some ROUND JAWS, perhaps from a good drill chuck, or a lathe chuck. I have found it messier, and leaves more marks, when clamping the petcock and remove the knurled part with a tool, than to clamp the knurled part in the vise.   The black outer cap hides your messiness, but why not do the job carefully, with thoughts of not marring the metal?

My present method is to use two pieces of BRASS or ALUMINUM, which is soft, and works well with the petcock knurling, in the vice jaws. It can be done so marring the knurled 'nut' does not occur.   NOTE that it is not uncommon to see a petcock that has not been serviced in a very long time, and has been, perhaps, exposed to moisture or near the seacoast, and the knurled ring nut will not remove, or not hardly easily.  I highly suggest you heat the petcock to near water boiling temperature, and then remove it from any heat source, and place the entire petcock into a container that has a reasonable amount of very thin warmed penetrating oil.  You want the penetrating oil to get into every part of the petcock and I work the lever a bit, then let it sit for a day, then work the lever again. I do this every day for about a WEEK.  After that, I can usually remove the knurled nut in my vice, using brass or aluminum protective jaw facings.
Some versions use a 'nut' with slots, and you can improvise for that.  I made a tool.

Pay attention to any indexing disc(s) inside, and how arranged.  Which side fits the other side, where and how the tabs fit, which disc goes back first, when re-asssembling.

Most of the time I have found the rubber discs with the 4 holes simply in need of a cleaning (with the rest of the petcock too), and then I reuse them, with a faint light...LIGHT!...smear of common dielectric grease (a silicone grease) on the rubber, and all the moving parts actually. There isn't anything in any of 'the books' about overhauling them, AFAIK.  Occasionally the dimple that locates the position has worn, I normally just forget about it, I can tell when the handle is fore, aft, up, or down!  You can reset the dimples though, with an appropriately shaped punch.  When re-assembling, push the body onto the 'nut', not the other way around! as the spring is very stiff.  I do it with the 'nut' again clamped in the softer vice jaws,........ otherwise you need leather gloves or your fingers will not like the knurling.

Always service the fuel tank and petcocks by REMOVING the petcocks from the tank. You NEED to see what grunge is there, clean the tank, etc.  

If your tank is empty, you can unscrew the large nut that holds the petcock to the tank, with a large Crescent or other adjustable jaw wrench, or wrench from your BMW tool kit.  They can be tight.  As you face the petcock from the side of the motorcycle, push the wrench to the left to loosen.

As explained much earlier in this article, that large nut tightens the petcock to the tank and at the same time it has a reverse thread on the petcock. Thus, when assembling to the tank, you start with the nut in an estimated position on the petcock. The idea is to END UP with about equal amounts of tank and petcock threads being engaged. 

****Once in a great great while I hear about someone with the wrong idea on how the petcock works.   As a general rule for all plumbing and piping, flow occurs when the handle points in the direction of the pipe, NOT when at 90 to the pipe.  Your petcock MIGHT NOT be marked ON, OFF, RES (reserve). It MIGHT be marked AUF and ZU. Please be advised that AUF does NOT mean OFF!! AUF here means MAIN tank portion is ON! This is not as laughable as you may think!    ZU means OFF.
Thus, when the handle lever is downward, the main tank is being used, when upward you are using the main tank and reserve, and when horizontal you stop the fuel from flowing.  HORIZONTAL IS OFF!

Some Clymers books are WRONG on the position of the handle for the choke (enrichener) ON CLAMSHELL AIR CLEANER MODELS.  That means pre-1980. When the clamshell handle is horizontal, the choke is OFF.  When the handle is downwards from horizontal, the handle points in the direction of extra fuel richness....towards the carburetor.   Clymers had this backwards!  Clymers wrong information has obviously caused problems with starting and running.

There are two tiny diameter plastic or metal pipes "straws" of UNEQUAL length going upward from the petcock into the tank. Once in awhile these are replaced due to cracking on the plastic ones or they are broken, etc.  The TALL one is for the regular main tank outlet, NOT the RES outlet.  The short one is just tall enough to keep bottom rust and water and other crap from getting into the petcock on the RES position. SOME have shortened it, by half is OK, it is NOT OK to remove it entirely. You will hardly gain much additional usable fuel, and you will get tank crap into things, including the petcock and maybe the carburetor. 

When the petcock is turned to the off, horizontal position, both tiny pipes are closed off from gas flow. When the petcock is in the main tank ON (down) position, fuel can flow into the tall pipe until no fuel is above its level.  Once the fuel in the tank (on THAT side for two petcocks tanks) is at or below the tall pipe level, you get no more fuel from that petcock in that main ON position. Putting the handle on RES (upward) will start drawing from the reserve portion of the tank, the short straw on that petcock, on THAT side of the tank on two petcock models.   Thus if you wanted to, you could run on one petcock, main (down), until the motor runs that side dry on the MAIN, then turn the other petcock to main (down) position, and then use two more RES (reserve) positions.   Up to you how you use the petcocks.  If you have a one petcock model, you have less options.   

The actual length of the petcock 'straws' themselves is not the same between all models. The dimensions I show here are PROUD OF THE SURFACE THE STRAW IS PRESSED INTO!
The straws can be metal or plastic.   The diameter of the straws is approximately 0.215"  (5.46 mm), but that varies a bit.  You can usually use 7/32" copper tubing available at hobby shops. If you need to, sand the end area diameter of the metal straw you purchased, and SLIGHTLY chamfer the down side very end ...both for ease in assembly and proper fit. The stock length (proud of the surface) of the SHORT straw, is 0.935" (23.75 mm); and 3.27" (83 mm) for the TALL straw.  Yes, you CAN shorten the short straw and gain a SMALL amount of usable fuel.  My advice is to NOT eliminate that short straw, certainly not below 3/8" proud of the metal.    You will need to add a small amount to these various lengths for the straw itself, as they are pressed-into the petcock.

For tank capacity, see your owners book.   Understand that the USABLE amount of fuel is LESS than the rated capacity. Few of you will be, or should, fill the fuel tank to the very brim, where no more can be added.  Fuel capacity is not what is in the books and this particularly so on the later tanks with the fuel restrictor metal flap in the tank filler area.
When BMW publishes a fuel tank capacity, you ADD the tank and reserve amounts to get the total amounts.   I mention this here because the literature has gotten rather corrupted on gas tank volumes. 

The two sides of most of the tanks do NOT hold the same amount of fuel for main nor reserve positions for two petcock model tanks.  These are NOT big differences.  I have seen differences of 5 to 8 miles of riding for the RESERVE.    I have not made quantity measurements.  You will hardly gain much, by shortening or removing the petcock short straws.  I am OK with a slight shortening, especially if you have aftermarket filters below the petcocks.
For high speeds I suggest BOTH petcocks be turned on, if two are available.  This helps with possible fuel flow due to bubbles, etc.   For an extreme case example, if you were running at wide-open throttle doing a high speed run, and managed to get to the rated 124 mph that some Airheads are rated for, you might need a gallon of fuel in 7 minutes.  You can measure the maximum possible fuel flow into the carburetors by (have fire extinguisher available!) themselves, in the following way and this measures the real world situation, via their own restrictions of the float needles and seats, etc.
Put a container for fuel under each carburetor, remove the carburetor bowl.  Turn on both petcocks and see if you can get 16 ounces of fuel into each container in 2 minutes.  That is the minimum required.  Yes, a cup a minute, or, around 1/4th liter.

Fuel flow of 350 or more cc per minute is adequate for our airheads.   I suggest that you have a MINIMUM of 8 ounces per minute per petcock.  Measure it by removing a carburetor bowl, and letting the fuel flow into a container.  Avoid sparks and fires!

Failure to continuously flow fuel can be a tank venting problem (or, with the fuel cap).  Especially on SHED models.


If the tank is not cleaned regularly, perhaps yearly, and especially if you don't fill up after every ride, or live in high humidity area, etc., the tank may accumulate a fair amount of WATER, which WILL ROT OUT YOUR TANK BOTTOM.   Water comes from the fuel itself, and most comes from inside the tank walls condensation, as the fuel and tank cools, when you park the bike.  This is particularly true in high humidity areas, where you can see dew on the tank, seat, etc., in the morning.  It is very important to clean the tank of water, or it WILL rust out at the bottom!

CLEAN THE TANK TOTALLY, at least yearly!!!  Clean the filters at the petcocks regularly; replace the aftermarket pleated paper filters when flow lessens noticeably.   If a fuel tank is very old, grungy, full of sediments, etc., I recommend using a high pressure car wash wand on the tank insides, then wash the tank and dry it.    Rust can be treated with phosphoric acid mixture (or the tank relined, which is a HUGE labor-intensive chore).

It is helpful to fully refuel before the bike sits overnight.  That helps with dew/moisture otherwise condensing inside the tank on the walls that would not be covered by fuel.  The water will condense into globules, fall to the bottom and cause the metal tanks to rust or rot out...yes, rot, even on aluminum tanks.    


Many folks add an additional aftermarket filter below the stock petcock.  I DO recommend that you DO THIS.   Sintered metal types are OK, as are paper types.  SCREEN types do very little good, as your petcock already has a screen.  That also applies to finer screens, which do only very slightly more.   I prefer the paper types, others prefer the anodized metal bodied internal sintered element types.  I like the larger Napa 7-02323 filter; but it is plastic.  The Fram G4164 will work fine too, and both are about the right size for most folks.  Be careful with the plastic filters, it is easy to crack the throat of these.  The FRAM is NOT made as nicely as the Napa filter, the Fram has less pleats, not well supported internally.   Napa has a 7-02357 that is smaller, POSSIBLY 'adequate' for dual petcock use.   The really teensy-tiny small plastic filters with the pleated paper elements are not so good, and CAN have some problems. Slightly larger ones are mostly OK.  These plastic bodied filters work best VERTICALLY, but are OK horizontally, but have less hose and vibration strain on the plastic if used vertically.  DO NOT fail to replace these filters now and then and do NOT fail to clean the tank, and clean the petcock filters now and then.

The Napa 7-02323 and G4164 are similar, and are actually replacements for early Volkswagen filters, VW ZVW262101....but are also used on many small garden engines, and so on. 
A.C. filter GF453 will work, but it really is too large, its diameter is nearly 2"; over-all length almost 5", and its stubs are single step with bump, and 1/4".  I recommend against this size unless you have to use it, or have a very deteriorating tank.

NOTE:  The Fram, and a few others, has a nice feature. The input and output stubs are dual diameter stepped, so fit whatever hose you are using.

NOTE:   The 7-02323 is very similar to the slightly cheaper Napa 3011.  The 7-02323 is part of the Napa Marine line of products, and has MORE pleats, and somewhat better made.  The 3011 has less pleats.  Either will work fine. 

 SOME filters have arrows pointing in the direction of fuel flow.  Sometimes filters do not have markings for which end is input, which end is output.  The input end, which goes towards your fuel tank petcock, is the end that fills the OUTER area, the output end is the connection to the filter INsides; but this is NOT critical and mostly just affects how long the filter lasts, from capacity (inside versus outside, inside has less effective diameter), and from tank liner flaking.

NOTE!  For whatever reason, Napa, and see, has changed its numbering system slightly.   If trying to see a photo or? of the Napa filters, if their search engine does not display a 7-02323, try entering it as 702323....and it may come up as SME-702323 or even SME702323.  Same for the 702357.  Don't know about  the 3011.

Note:  Other filter numbers that seem fine are:   Baldwin BF-989; Wicks 33-027; Those two are METAL filter bodies; there is also the Wixom 33011.


Reasonable quality aftermarket in-line filters do a vastly better job than the in-tank screen, which is ALSO NEEDED; whose purpose is to capture LARGE sized junk. The aftermarket external filters are a big help because it takes hardly anything as thick as a human hair in the carburetor to cause problems.  A few aftermarket filters have cleanable elements.  Note that if fuel flow decreases enough, your engine will run lean and potentially hot, might stumble some, and a lean running engine can be harmful to your bank account!!   

If you install aftermarket filters, do carry some short piece of hose, clamps, or whatever YOU need, to enable you to remove a filter if it should crack and leak, or, you have some other problem.   PLEASE be careful to do a neat installation, and think about the long term.  You do not want a faulty installation allowing gasoline leaks.  In other words...whether you use these aftermarket filters or not, consider what might happen, 300 miles from nowhere, if you have a leak in a rubber hose...or a filter leaks, breaks, whatever. Carry a piece of hose, perhaps a clamp, perhaps a way to plug one side if you have a dual petcock tank.

Fuel hoses:

Sometimes you can pull the hose off with your hands without excessive force, but often they really stick well.  The official BMW hose, whether the old silver braided stuff or the newer black metric gas line, is slightly less in OUTSIDE diameter than American fuel line...and, being smaller, fits the cross area of the airbox. All fuel hose of any type, but especially rubber types, tend to stick to the outlet nipple pipes (besides making a good fluid seal), and no hose clamp is needed, except with SOME aftermarket in-line filters.  Do NOT use ANYTHING but GAS/FUEL line!   Some folks put a pair of pump-pliers onto the hose near the petcock and give a mighty pull. That tends to tighten the hose! it stretches the rubber, which is reinforced with fibers....just like the old Chinese finger-pull toy.  It also can crack or break the plastic outlets.    Better to use a very broad flat tool of some sort and push the hose at its end, whilst gently pulling on it. If replacing the hose, cut it off.   I got pretty anal about all this once and made a tool to remove hoses.  Was just a U-shaped piece of metal.  Another idea is to put a large diameter 'fender washer' onto the outlet nipple before pushing the hose onto it...that way, when removing the hose, you both push on the washer whilst gently pulling on the hose....and the hose almost always easily comes Chinese finger toy problem here.  BMW does not use fuel line clamps on Airheads.

The best fuel hose I have ever tested is Tygon type F-4040A.

One that did NOT pass my tests (various solvents, gasoline components, ETC...short term and long term) is Masterflex "FDA Viton" 96412-D.  VITON of any sort is NOT recommended by me.


One should regularly drop the carburetor bowls and empty them out, even flush the bowl jets, pipe, etc., with something STRONG, like Berryman B-12 CARBURETOR AND CHOKE spray, and replace the bowl gasket now and then. I recommend emptying the bowl of junk, water accumulates there, every few months. Junk can rise and get into the idle pilot jet, which has an exceedingly small hole....and junk and water can mix and tend to plug the tiny corner well bottom jet, and then the enrichener (on the CV carbs) won't work properly.  A bad bowl gasket, especially at the CV enrichener down pipe area, can make your enrichener (choke) work BADLY.   The central jet assembly in the CV carburetors has a tendency to collect black grunge...remove the entire assembly and clean it now and then...and be careful upon reassembly not to overtighten, nor tighten the assembly against the needle tip....!!!.  Parts fit properly only one way.


Fuel caps and venting methods for the fuel tank varies by year/model, but it is common to have Airheads run out of fuel.... stumbling, loses power, etc......and the problem will be the tank vent...or the cap.  This is provable by loosening the cap and hearing a whoosh of air entering (if you CAN hear it), and the bike then runs OK within 15 seconds.   Stumbling from this vacuum problem usually happens much more quickly with a more filled tank and also from higher speeds.   Fuel cap cork seals are replaceable.  So is the black surround on the later caps.

From the /7 onwards, BMW changed to a screwable, removable, fuel cap.  They are NOT all alike!  Even ones that look alike may not be, and this goes even further with foreign country shipped bikes!  My coverage here is general, for USA caps, but most applies to foreign.
The caps with the BMW emblem on top may not be the same internally, which affects how you go about drilling/locking/ETC.

The late 1977 (and probably as late as 1979 in many instances) gas caps are changed in design, so that they allow air to pass to the inside of the tank, but fumes are not supposed to pass to the outside of the tank.    To accomplish this, there are valves built into the cap.  These years of caps could have problems.   The symptom of this particular venting problem is when the bike starts running very lean, may buck and seem to run out of fuel, and this typically happens after some time on the highway, the time can be shorter if the tank was rather full to start with.    Another problem, one that was also addressed by a SI from BMW, is that in extremely hot weather the cap venting is inadequate, and there may be fuel problems.   A 5th hole was added to the cap, see sketches below.  That 5th hole was added in production after certain bike serial numbers.

OPENING the cap (unscrewing it a bit) will 'fix' the fuel flow problem within 15 seconds, and this 'test' is nearly 100% for a bad cap vent.   The cap can be modified to fix the problem permanently.  Put it upside down on your workbench.   Drill AWAY from the center, any direction, about 1/2" from the center.  Thus the hole you will drill is off-center.   Drill, 3/32", from the bottom of the cap, through the metal shell, and continue drilling carefully....go through the air space, and drill into the softer material that you then feel you are drilling is maybe 1/8" thick. Drill through it, and then stop drilling.  Clean out the cap as best you can of drilling swarf.   Replace the cap on the tank.  That's all there is to this.

NOTE:  Oak uses different dimensions on this, than I do.  His description is a bit different also.  Oak says to mark the center, as I do.  Then he says to use ~1/16" drill bit, and drill at a point 5/16" outwards from the center, anywhere's on the bottom.  So, it looks like my sketch, below, just not drilled so far outwards.   Oak gives the following instructions:  Lightly centerpunch the spot, then drill carefully through the metal bottom. He notes that the drill will go further into an empty space internally within the cap, then strikes another object inside the cap.  That is the lock and vent area.  It is plastic. Drill further into that, CAREFULLY, until you feel the drill has gone through the next internal plastic layer (about 1/8").  THEN STOP.

***Although the above methods will work, the official factory method is a bit more detailed.  The method used by the factory, and for the upgraded caps, is to locate the center (with reference to the 4 existing holes, that is) similar to as in the sketches below, but the hole is drilled IN THE CENTER, where the photos show the cross marks.  BMW specifies the hold as 1 mm in size, and that EXACTLY a depth of 22 mm, and NO MORE, is to be drilled (to avoid the key lock).
1 mm is a bit larger than 1/32".  You can use any size drill near to 1 mm.  DO hold to the 22 mm, which is 0.866".

No matter which procedure, blow out the cap with compressed air and maybe a solvent.   The original key will still work.

NOTE that these caps are NOT to be used like car caps!  AVOID spinning the bike caps to the ratcheting point when tightening them!!


The fuel caps are not the same on the last Airheads; these had a venting system for the fuel tank that vented, via electrically controlled valves, the tank fumes to the crankcase. The caps had both a pressure and vacuum release built-in. However, the problems with caps being unable to be removed is similar between all these SCREW caps.,,,,see next section.

Rubber seal for the above screw-in gas caps.  This is NOT the black trim ring, but the SEAL, originally RED (which broke after awhile):  51-25-1-453-148.

The black trim ring is 16-11-2-307-360.  This is NOT a seal, but a trim piece, and it also helps keep rain from getting into the 'shelf' of the tank under the cap (where a drain hole is).

If you are having venting problems caused by a faulty fuel cap, on one of these later Airheads, that have the SHED system (solenoid valves in the starter compartment, connected to the fuel tank, etc); then, since the caps are $$$$ for the SHED type, and $$$ for the non-SHED type, you might consider this, posted by Tom Cutter on the Airheads LIST on August 26, 2013:

............take off the SHED cap, put it upside down on the table and locate the center of the cap (underneath).   Drill a 1 mm hole in the steel cap. Push the drill bit down about 1.5" until you hit metal again, carefully drill through THAT metal and stop.  Voila! Instant non-shed cap. Never have another tank vent problem.  Tom Cutter Yardley, PA

Ratcheting and unscrewing and re-keying:

This is a separate section; and you should NOT use the above two photos; which are ONLY for venting problems with the earliest caps.

There are THREE basic types of these screw-top caps.
The earliest versions have a anodized finish and a BMW LOGO......the later ones do NOT have that BMW logo on the top. Both caps have a replaceable plastic 'rain skirt', that fits into a groove in the cap cover part.  The last type of cap has vacuum and pressure valves inside, and are used with those Airhead models that have the fuel tank vented via a electric solenoid valve to the engine crankcase (and also have positive fuel shutoff via an electric solenoid valve).

It is possibly better to not key-lock your screw type fuel cap and certainly best NOT TO TIGHTEN THEM TO THE RATCHETING POINT. The ratcheting is NOT the same as car type caps, and is NOT to be used for the same purpose, which on cars is to ensure they are tightened fully!   Frankly, I DO NOT understand why BMW designed the cap the way they it is not the same function as other vehicles' ratcheting caps.    If you regularly tighten the Airheads cap to the ratcheting point, you will eventually strip the ratcheting parts and have fun removing the cap.   I understand you may want the cap locked.  If you insist, then tighten them to just about ratcheting or one ratchet.  The BMW ratcheting gas caps will wear out and cause you problems if you insist on ratcheting them a lot.  The plastic pin jumps into and out of 4 holes when you do that, and wears it badly.    IF you wear the ratcheting mechanism enough, and this IS COMMON, the cap may then seem totally non-removable, and just spins in place.

There are several methods of removing the caps when you have that pay attention!

1.  ANY of these caps can be DRILLED downwards through the TOP, and then you lock the top and bottom sections together with some sort of 'tool'.  This drilling can be done with a 1/8" drill, and you then use a long shank from a 1/8" pop rivet...or, even a nail.  If you drill in the handle area, or at the hinge area, the drilled area will not be seen, and the cap can be left that way,...the only problem, if any, is that the lock won't work.   You can drill (NO electrical sparks, please!) through the LOGO if the cap has one, or, under the handle or hinge area.  About 7/8" ...or 15/16" ...from the center of the cap is about the right distance.  You can drill under the wide area of the handle, and countersink the drilled area, and use a long countersunk head screw as the method of locking the top and bottom of the cap together.     If you drill the cap downward, through the top, into the metal, through the open space, and down through the lower metal, you will need a several inch long drill bit.  My suggestion is that you drill the cap with a drill size that fits a nice convenient nail, and you can use the nail permanently in the cap, or use it as a new type of cap key.   This is NOT the drilling method shown earlier, for venting problems on the earliest of these caps.

2.  If your cap has the BMW LOGO, and you do NOT want to drill it, you can grab the plastic skirt with pliers, and slowly pull it sideways away from the cover.....keep pulling (it stretches), until clear of the cap and lift and pull until it comes off.  You will now see that in the groove that this plastic skirt fit into, there are THREE holes, 120 apart.  Take a nail or piece of broken coat hangar wire, etc., and insert a bent tip end of that into one of the holes, and lift on the cap cover, whilst gently turning it and pushing the wire inwards.  You are 'feeling' for a dimpled area, and when found, you have the parts temporarily locked, and can UNscrew the cap.
Read and look carefully at this article:

3.  If your cap does NOT have the LOGO, and you do not want to drill it, the body ridge snaps into a cover groove, and you might try pulling the cover off the body.  

4.  One method of getting the cap to unscrew, is to wrap a very strong belt or rope around your waist, through the cap handle, and stand on the pegs; and, holding with a quite strong upward force, try to unscrew the cap. Don't go too strongly on this, unless your purpose is to try to break off the top portion, not just unscrew the cap.  I suggest that you use a fair amount of force, but not handle-breaking type of will also protect the tank from bending.    With the LOGO cap, the cap cover has a rolled metal edge, is very strong, and you might bend the TANK before the cap pops off (that happens under extreme force). 

5.  If you can get the cap off (without drilling from the top), then there is another method to use.   Drill, a hole, through the SECOND metal layer.   That means you drill through FROM the bottom of the cap, and continue until you drill through the next layer.   The size of that hole is unimportant....well, relatively.   Hole of about 3/32" is OK, although smaller will also work.  You can now lock the sections together and you have not drilled to the top of the cap.

Here is a hyperlink to Anton Largiader's website article on the caps; read it to further your knowledge and UNDERSTANDING:

Re-Keying:  see the above largiader link; and, read below:

NOTE!!....the information is rather limited.   For a fuller treatment, with photos, see the BMW Riders Association publication called On The Level (OTL), dated April 2006.  That has an article by Anton Largiarder that is quite informative on the later screw caps....including information on changing the locks, ETC.   This information MIGHT also be on Anton's that article...and also see mine:

RE-keying the fuel caps; lock cylinders, purchasing caps/locks/keys.....and more .....:
my article #75, especially item #7 in that article,  for LOTS more gas cap lock information:  

The 1977+ fuel caps (these are the ones that screw into the tank) create lots of questions at times.  Here is some information, but you may well have to ask your local BMW dealer about the present status of these items.  Frankly, you may need to find a parts-person willing to spend the time with you on this, particularly if you want to order a cap with a key that matches your other bike key(s).    It is possible, sometimes, to carefully remove the old lock cylinder by drilling it out and removing the bits.  This has been done in various ways, including doing it mostly downwards, then adding a sheet metal screw and prying the lock upwards. 

Be sure to read this article:

Usually, folks want their 'new' cap to have the same key as their ignition/bags key.  This can't be done on early caps by the normal means of re-keying as done with the bags, as the fuel cap lock is a plastic affair.  I would not be surprised if BMW changes the design so it CAN be rekeyed by the normal method of taping the body (keep pins and springs from flying), inserting OLD key instead of the new key, and filing down the protruding pins (NOT the last, END, locking pin!!)....then inserting the lock unit.  For the cap with a LOGO, the cover is held to the body by a crimped can undo that crimp, remove the body, and with a new cap, pry the body out of the cover, and extract the plastic lock cylinder assembly.  By thinking this over, you can see that you can transfer an old lock cylinder to a new cap. 

When a key is in the lock, and turned, you can see a small release point in a small opening in the plastic.  PUSH that part inwards with a tiny tool, and then you can withdraw the key lock assembly!   If the key lock assembly is not broken, cracked, etc., you can clean it, lube it with some light grease, and insert it into your NEW cap, by aligning it and pushing it into the MIGHT need to turn the cap via a bit of force, to push it all allow the lock to fully install.  Once installed, it is permanent.

Some caps have smog/vapor 'enhancements'. Some remove the vapor items entirely, and plug the vertical pipe in the starter motor area.  Some remove the tank flapper valve, some remove all the solenoids, etc.  YOUR choice.  The caps for that system are different....and say SHED247.   Do not use the wrong cap, unless plugging.

The black standard cap is 51-25-2-307-125
The plastic trim ring is not included with caps, and is 16-11-2-307-360
The standard cap with a lock and a key is 51-25-2-307-168
The SHED caps are available in both black and chrome.  The black one is 51-25-2-307-140; and the chrome one is 51-25-2-316-185.  You won't believe how costly all these parts (except the ring) are.

A new lock cylinder is 51-25-307-166.     The key will not fit your other locks.    The original keys for the motorcycle included stamping of a number on the key, a knockout part that is; and also a small tag came with the keys, with a number on it.  If you supply the number to the dealer, he may be able to order a custom-keyed cap for you.  I say MAY, because you have to have a knowledgeable parts person; AND, this number may no longer be available from BMW.   Custom keyed cap:  51-25-2-307-173.    A new cap with new cylinder and 2 keys (obviously not keys that match your other bike keys) is 51-25-2-307-168.      Caps without lock cylinders are no longer available; unless a dealer happens to have one.   

Because the cap internal plastic pins wear from misuse (ratcheting), I do NOT recommend even trying to re-key an old cap!!!  I would ONLY get a brand new custom keyed cap, which will take time to order and get it, or just get a new cap with whatever lock and keys it comes with.   Custom caps  used to cost the same as a cap with lock that had no common keys to what you already have.  There is also the transferring of the old lock, you can also consider that, but consider the details.  

TANKS, SEATS, fitment, differences, capacities, etc:

It is not at all uncommon to swap various tanks and seats.
The /5 and /6 SEATS are LONGER than the /7 seats.
The /7 SEAT will be OK with a /5 or /6 tank.
The /7 tank can fit a /5 or /6, but modifications are needed, otherwise it is rather ugly.

There are differences in how the various seats mount.   Early seats had the hinge portion screwed to the seat, with allen screws, and if they rust-out, can be a bear to remove them....making it a bear to get the seat off the bike.  There are variations.
It is not uncommon for the area around the seat portion of the hinges to rot out, from moisture accumulation.  This can be fixed by making up small flat plates in a U configuration, and having them welded to the seat bottom.  This CAN be done with the upholstery intact, if wet cloths are used.   It is a good idea to drill the recess area of these hinges in any event, whether during welding repair...or allow any accumulated water to go downwards.   Use antiseize compound on seat screws.

Don't depend on factory printed information on the tank capacity.

Because BMW has a month-long vacation shutdown (the factory is closed in August), a year model could have been produced at the end of the prior calendar year.   

In 1970 and 1971 there was available a larger capacity 6.3 (6.0 per BMW) gallon tank, it was special orderable for the 1972 production year...and a few, I think, were produced that way for 1973. 
1972 had the smaller capacity tank, almost exactly 1 gallon less capacity.
The 1970 and 1971 fuel caps had the hinge at the FRONT; but in 1972 and 1973, it was at the rear.
The 1973-1/2 (when BMW also switched to the long wheel base) had the cutout underneath for the hydraulic master cylinder that appeared later, actually in the 1974 /6 model.
The standard capacity 6 gallon tanks had black knee pads.
In 1972 the Toaster tank was, however, standard for the U.S....withOUT pin striping.
Early in 1973 production there were not only the toaster chrome panels, but also pin stripes.
Rubber pads were available for the small tanks...AFTER the chrome panels were discontinued.
Authorities tanks (Police) look like the /6 tanks, with the rubber pads too....but the top has a lid.
/5 tanks have screwed-on Roundels.

When BMW publishes a fuel tank capacity, you ADD the tank and reserve amounts to get the total amounts.   I mention this here because the literature has gotten rather corrupted on gas tank volumes. 

10/05/2003:  incorporate all previous changes & updates on the petcocks & cap removal methods
03/30/2004:  spelling typos, emphasis
03/31/2004:  Final version; add Fram filter number, minor clarifications, eliminate SOME redundancy
04/04/2004:  add overhauling Everbest information.
07/01/2004:  Slight updates, some clarifications
08/26/2004:  update for venting, modifications, tanks, seats
09/07/2004:  add tank capacity information
09/12/2004:  updated, added links
10/01/2004:  fix waste...waist typographical error
10/29/2004:  update numbers, clarify caps and keys details
03/05/2005:  Revised extensively to have information in better order, as things had been overly-repeated
                    & much of the text was choppy.  Added some comments on filters and the gas caps.
11/09/2005:  red note on Napa filters numbers.
03/23/2006:  more information on the Napa filters.
04/18/2006:  Revised, for clarity, information on the gas caps; minor in other areas.
11/07/2006:  clarifications and emphasis
11/21/2006:  add roundels information
02/01/2007:  Remove Roundel information, as it will be updated and expanded in article 68.
07/28/2008:  Revise a lot of the article strictly for clarity.  Also add not on Anton's article in OTL.
02/01/2010:  Add photos of gas cap drilling for venting, re-arrange article some, and add hyperlink
11/17/2010:  Add information on petcock straws
06/01/2011:  Clean up article, mostly for clarity reasons
06/04/2011:  Add /5 petcock sketch
04/03/2012:  Small updates, including adding Anton's article as a link
04/04/2012:  Clarify and emphasize details on cap drilling types and methods and why/what
07/21/2012:  Add a bit of information on my testing of fuel hoses, and my recommendation of
                    Tygon F-4040A
08/28/2012:  Add to the petcock overhaul section.  Add QR code.  Change Google ad & meta coding.
11/14/2012:  Totally revise, clarify, eliminate duplications, expand some areas.
12/01/2012:   Add DIRECT link to Vetch's article
04/15/2013:   Add part numbers and information on screw cap trim ring and gasket.
08/26/2013:  Add info from Tom on drilling SHED cap
08/06/2014:  Add information from BMW bulletin on the cap drilling.

Copyright, 2014, R. Fleischer

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