BMW motorcycle Fuel Filters, Petcocks, fuel caps

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

fuelfltrs&petcocks.htm-1

Petcocks are fastened to the tank by right and left hand threads on the petcock versus the tank. The Petcock large nut has to be engaged the proper amount onto the petcock, before then screwing it upwards 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 but 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), 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.
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.

These 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: http://benchmarkworks.com 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.

What follows for the rest of this long article, is Snowbum's exclusive input:

NOTE! On these /5 bikes with the above petcock,
the CHOKE cable tends to rub the underside of the bullet area.
It can, and has, worn through the petcock!
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 it...it 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.

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 is necessary whenever the screen is removed. Stated differently, you 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 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 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, but does have 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. 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.

***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!!...to 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?

Some versions use a 'nut' with slots, and you can improvise for that.
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. When re-assembling, push the body onto the 'nut', not the other way around!

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.

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.

FUEL CAPACITY and FLOW:
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. Measure it by removing a carburetor bowl, and letting the fuel flow into a container. Avoid sparks and fires!

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. 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 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 www.napaonline.com, 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 adequate are: Baldwin BF-989; Wicks 33-027; Wixom 33011.

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 filters work best VERTICALLY, and have very much 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.

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!...as 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 off....no Chinese finger toy problem here. BMW does not use fuel line clamps on Airheads.

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 an early Airhead 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 late 1977 (or perhaps from 1978) 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. The very first of these caps could have problems. The symptom of a 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. OPENING the cap (unscrewing it a bit) will 'fix' the 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 through....it 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.

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 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, and the red ones 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).

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 have never FULLY figured out why BMW designed the cap the way they did....as it is not the same function as other vehicles' ratcheting caps. If you regularly tighten the Airheads cap to the ratcheting (really a beyond-locking-point) 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 problem...so pay attention!

1. ANY of these caps can be DRILLED, through the TOP cover, 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 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.

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 force...you 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 get the cap off without drilling from the top, then then there is another method to use. Simply drill, startia/u> a hole, through the SECOND metal layer. That means yo9u 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.

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 website......read 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: locks_caps_etc.htm

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.

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 stock....to allow any accumulated water to go downwards. Use antiseize compound on seat screws.