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Cylinders (iron, steel, Nikasil, Galnikal).
Boring, honing, cylinder shims, plates,
gaskets, o-rings.
Removing cylinder shims, ridge removal, measuring
cylinder taper, installing new rings, etc.

Copyright, 2013, R. Fleischer
article 60, sub-section 3

For information on the cylinder studs & Helicoils:
For information on pistons, rings, etc. :


BMW has used both Galnikal & Nikasil in describing their late cylinders, & never explained the difference.   BMW used at least two companies in producing these cylinders for them.  They are basically the same process.

Nikasil is a registered trademark of Mahle Gesellshaft (yes, the piston and filter makers).  Coated cylinders are done via a plasma process, in Stuttgart, & is a blend of nickel & silicon carbide.    Galnikal is a trademarked name for the process used by Kolben Schmidt, which is a major German foundry & castings maker.  They also make pistons.   If you have Kolben Schmidt cylinders, or other products, they use a symbol which is a letter "K"  on top of a letter "S", and it looks like this:


>>>As far as this article is concerned.....and, for most purposes anyplace, cylinders are referred to as iron; or Nikasil (even if Galnikal).

So...just what is Nikasil or Galnikal? 
While a coating can be applied to iron or steel sleeves that are pressed or cast into cylinders...& I list companies reclaiming old iron cylinders in my references article, generally speaking, when one says "Nikasil Cylinders", that means that instead of an iron sleeve, cast or pressed into the cylinder barrel (early barrels might be cast iron), the Nikasil/Galnikal barrel is a one piece aluminum alloy, precisely manufactured as to roundness, taper, etc.  A very uniform & very hard coating of, typically, nickel & silicon carbide is put on the inside surface, by means of an electro-deposited coating.  That 'plating' is NOT very thick, about 0.003", & the silicon carbide particles themselves are about 4 microns in size.  The result is a light-weight cylinder with an extremely long wearing surface, that the piston & rings work against.  Because of the various characteristics, the cylinder (very precisely finished to begin with) will strongly tend to NOT change its shape, that is, the inside bore does NOT tend to get oval or get other irregularities.   Baring a mechanical problem that injures the thin coating, the cylinders will last nearly forever, & only rings, which wear FAR faster than the cylinder coating, & sometimes pistons, are replaced at fairly high mileages.  The Nikasil process was originally developed for race cars, & now is quite standard on many cars & bikes.    It is this type of cylinder manufacturing that has led to vastly improved mileage before necessary overhaul, in modern cars and motorcycles.  

The cross-hatch pattern of these cylinders is VERY precisely controlled, & may well look almost original, even after many tens of thousands of miles.  A precisely done cross-hatch is one key to good ring break-in (controls ring rotation too, where required!) &, ESPECIALLY IMPORTANT, good oil control.  The cross hatch valleys, which are easily seen but not as deep as you might think, are what retains the necessary microscopic amounts of oil for lubrication.  The coating is what is called oleophilic. Nikasil cylinder bores are VERY dimensionally stable & have very low distortion. The microscopic pores retain the oil in the crosshatch valleys, so the bores can be finished to quite close tolerances.  Nickel silicone carbide coatings are VERY hard, VERY wear resistant, have LOW friction, & the surface hardness is about 90 HRc. 

Nikasil is not the only type of coating used for cylinders.  Many other types of extremely hard coatings are used, usually carbides of some sort.  Rings must be of appropriate types to match the type of coating!   In many if not most instances the pistons are identical with other types of cylinders, including the non-plated types.

It is generally...and wrongly... accepted that one can not successfully bore a Nikasil or Galnikal cylinder. There ARE companies who specialize in the necessary process. There are companies who repair damaged Nikasil cylinders, and also successfully bore iron cylinders & then apply a hard long lasting coating.  It is my personal belief that the cast-in-place sleeves of the pre-Nikasil cylinders are not dimensionally as stable; and, in particular, the old R100 cylinders are rather often bored, and do not work all that well after boring, due to thinness/distortion.  If boring is to be attempted, special pressurizing plates must be used to try to keep dimensions stable.   Sources for cylinder repair are listed in on this website (under C for cylinders). Sources are listed that can bore/hone & even apply Nikasil-type coatings to not only your damaged Nikasil cylinders, but also to pre-Nikasil cylinders.  

Plating the old cylinders often works, but it is iffy on the old R100 cylinders, only a specialist with a lot of experience is to be trusted.  Except for some sort of damage, Nikasil, Galnikal, etc., coated types of cylinders generally do not wear in normal use enough to talk about, & should not generally be honed in the classical honing sense when one does a rings replacement job.   If you install the wrong type of rings, you can ruin the cylinder coating!   There is no difference in the pistons (for Airheads), the difference is in the RINGS.  Be sure the company doing the cylinders understands the BMW piston to cylinder distance specification. If this is not made clear, they will have to redo the cylinders (because YOU WILL have problems), so tell them it is for a BMW bike, & that they must use the smaller clearances (see BMW manuals, or this website).

If you intend to rebore an early cylinder, the VERY BEST method is NOT a boring bar type of tool, but to go to a quality & knowledgeable machine shop; have the INSIDE DIAMETER GROUND on a special type of grinder.   One of the cylinder restoring companies, that will apply a Nikasil coating afterwards, would be my choice. It is entirely possible that the cylinder restoring company can do the entire job!

So, to make this clear: 
If you intend to bore an old-style cylinder, the best method is to use an inside diameter type of grinder, plating, and then a final honing.  That is tricky work. I generally have, especially in the past, recommended HONING of the R100 cylinders to size; or, replacing them.  Some of the cylinder restoration companies are very good, however, with boring/honing/plating.   The Airheads smaller than the R100 can just about always be bored, honed, etc.   This is a bit complicated to explain this early in this article about which sizes, pistons, etc.    NOTE that Nikasil cylinders, complete with pistons & rings, are available as sets, and while they cost $$$, they almost always work perfectly the first time, & you do not have to deal with problems with improperly done boring/honing, ETC.    Thus, if you have a bike that will accept Nikasil cylinders, I have long recommended that you purchase the complete set, with pistons, ready to install.   If you wish to have R100 cylinders re-done, go to a good recommended cylinder restoring company; have
the cylinders coated; use the appropriate rings. DO tell the company that you want the tighter specifications for clearance, that BMW uses.

The R100 engine to 1980 used steel lined cylinders, NOT the Galnikal or Nikasil coating of 1981 & later cylinders.  These R100 liners are very thin & usually can not be successfully bored to the 0.5 mm oversize....they would be too thin.  Pistons & rings were offered at 0.25 mm oversize.  If these cylinders show a lot of wear, one can probably just use 0.25 mm oversize sure the end gaps are proper, if not, grind them so they are within specifications.   I am aware that purists might not like these ideas.

In the later 1980's, BMW added a "step, etc." machined into the cylinder spigots, & a similar matching step was in the crankcase opening.   If you try to fit late cylinders to an early bike, you MUST machine the cylinder bases.  This has nothing to do with the 97 to 99 mm spigot change in the /6 era.

BMW does not offer oversize pistons for the Nikasil cylinders. 

BMW end gap specifications are a bit tight, looser is a bit better.  The smaller engines have enough material to be bored 0.5 mm over, so 0.25 oversize rings are not offered, only 0.5 and 1.0 over.  These do not work well in worn cylinders...tend to twist & cause problems...even piston or ring breakage.   Some will get away with doing it.   Usually, this type of usage will cause rapid wear of the cylinder.    NOTE that boring is rarely successful unless the proper equipment....including alignment/holding plates... are used.  There is no basic difference between pistons used with Nikasil, and with steel or iron cylinders.  THE difference is in the RINGS materials.   I have had rather good results using new or oversize rings (grinding for proper end gap) even on cylinders with a considerable amount of taper, etc....this is on the older steel CYLINDERS (and the iron ones of the /2 era).  For Nikasil cylinders, BMW does not supply oversize piston rings; best to not try to modify end gaps on rings to make them fit.  You could find proper aftermarket rings, fit them, even find oversize pistons, etc.  Just because BMW does not offer things does not mean you cannot find some company that does.  Be super-cautious if not using BMW-supplied rings for Nikasil, the wrong type of ring material can destroy a Nikasil cylinder.  Basically, iron rings are used, never chrome rings.

So, what about failed Nikasil cylinders?
While, as mentioned earlier, boring is not generally done with the Nikasil or Galnikal cylinders, it can be done; and I have the specialists listed in my references article, under C for cylinders. The problem then is to find aftermarket pistons that fit...unless cylinders are done to match BMW pistons.  Some, if not most, early cylinders can be reclaimed by plating them with a coating that is similar to Nikasil or Galnikal.   

Boring, in general:
Many places rebore cylinders.  For our Airheads, it can tricky to maintain concentricity & avoid taper.   I recommend against BORING R100 cylinders.  They should be HONED to size; IF that can be done.   Honing & boring, in any case, should be done using pressure plates.

Improperly done boring?   You will find your engine is using a fair amount of oil. Inspection of the inside of the cylinders will show areas where the rings are not contacting properly; those are always at the places opposite where the cylinder studs go by!  The boring of the steel cylinders (non-Galnikal, non-Nikasil, ~pre-1981) is tricky, and if not done with proper equipment including a thick plate, will not work well.  Altogether too many re-bore jobs end up being junk.  Might be best to just get new cylinders.  IF you are going to re-bore cylinders, my first choice would not be conventional boring, but what is called inside grinding, done with great care.  They can then be finished with honing in the normal manner.  Coating with something like Nikasil should be considered.

Bottom line:  Consult your favorite independent BMW shop, such as Ted Porter's shop.

Honing is always done on the old iron & steel cylinders, but NOT actual true honing on the Nikasil/Galnikal cylinders (honing may be done prior to reclaiming a Nikasil cylinder prior to plating).  

Finish honing is best done with the SofTool brush hone made by Osborn International.  Ball honing is adequate but not as good, as the ball tips wear after awhile and the hone does a poorer & poorer job.  The SofTool does not exhibit such characteristics.

The crosshatch pattern is very important for ring wear, ring rotation, & even piston groove wear.   The ideal proper angle is about 28.   If the angle is too shallow, the rings will rotate too slowly, the engine will use oil, the rings might skate.   If the angle is too steep, then starts can be rather dry, with fast ring and piston wear.  The common 45 usually is OK....but I DO think that 28 is better.   Usually 220-280 grit stones are going to be OK.

If you must, or want to, hone a Nikasil/Galnikal cylinder, I suggest you use ONLY the "Brush Flex-Hone" which is also called a Ball Hone.    Use ONLY the aluminum oxide 240 hone.  Do NOT use the silicon carbide hone 320, as there can be problems with that fast cutting hone.  The purpose of the honing, IF you want to do it, on Nikasil/Galnikal, is simply to deglaze and clean up.  Be exceedingly careful about using the 240 grit hone.  I try to avoid using such & might consider a much finer grit...perhaps 600 or higher...BUT!.... NOTE WHAT I DO AND DO NOT:
I do NOT hone Nikasil except in heavily glazed situations (very high mileage primarily).  Even then I may not.  I simply use 3M green industrial pads, soap and water, and my hand.

For recommendations & information on repairing, boring, & plating cylinders, see ...under C.

The original 'squish' pistons are no longer available.  One can modify the head & cylinder (deck height change of about 0.050")  and thereby get your own squish pistons in practical use.  Check for lots more information.  The 900 cc model's pistons are pricey.  Reclaiming the cylinders to allow use of those pistons is something to be considered.

Cylinder Base Shims (also called gaskets by BMW):
((occasionally these shims/gaskets are called compression plates))

There has been a lot of confusion over the cylinder base shims (some of which BMW calls gaskets).  Some of the confusion occurs because folks do not know what shims or gaskets were originally used on any particular model & year, & confusion persists due the following things:

1.  Bikes were shipped by the factory with compression lowering shims, mostly to countries with poor octane rating of its gasoline's. 
2.  Bikes were shipped by the factory with different pistons, giving different compression ratios.
3.  Mid-cycle changes were made.
4.  Owners & dealerships have made modifications. 
5.  NO paper gaskets have been used BY BMW themselves.
6.  Different bore sizes mean different effects by the same shims, even with same piston shapes.
7. BMW themselves confused things by making errors in their own parts literature, and even in Service Information bulletins, AND, BMW's literature folks PROBABLY made errors in translations.

Some item mentions in the below section may be controversial. It is, however, my take on all these shims/gaskets/etc.  You do NOT have to use any specific STOCK arrangement, & you may have good reasons not to.  If you remove a shim/gasket from a motorcycle that has previously used it, & have no intention of replacing it, you MUST remove any wear ridge at the top AND ESPECIALLY TOWARDS THE BOTTOM, just below where the prior oil control ring stopped at BTDC. See the end of this article on specifics on how to/not-to, use ridge reamers, etc.

I will use GASKET, SHIM, & COMPRESSION LOWERING PLATE, NEARLY INTERCHANGEABLY IN THIS ARTICLE, EVEN THOUGH THAT IS NOT STRICTLY PROPER.    The cylinder base gaskets/shims has some erroneous information printed here & there, possibly even on some on-line fiche.  The original information was updated & then I found them again on the 1982 microfiche....the earliest real microfiche (plastic photo film) I own.

I identify the parts & part numbers, what fits what; &, what was stock.  BMW offered various shims/gaskets & NOT all were shipped as standard to any one country at any one time.   BMW offered two shims/gaskets (actually were 3, one almost never seen) that fit the R75/5 for lowering compression ratio but they were not STANDARD, unless requested.

The stock cylinder base SHIM, during 1970-1975, was 11-11-1-255-001, it was aluminum, & 1/2 mm (~ .020).  This was used for all the /5 models, & was used on all the /6 models up to 08/1975, with possibly a few being installed by the factory somewhat later. 

To reduce the compression ratio one point (1.0 in compression ratio), a thicker gasket is used, 11-11-1-257-092, which is 1.2 mm thick (0.047") and has a 97 mm hole.  It is used IN PLACE OF  -001 metal gasket mentioned in the above paragraph, WHEN compression lowering was desired.  There is some confusion if you look at the parts books, etc.   Long ago there was a 1.0 mm thick gasket, & there were literature mentions that were sloppy about the thickness, being only approximations.    This -092 gasket is sometimes called the "96ROZ" gasket in BMW literature.   It was intended for use up to 09/1975.  It supposedly reduces CR from 9.5:1 to 8.5:1, but it cannot do that equally on all models, because it was for the R50/5, R60/5 & R60/6...and the R50 has slightly smaller bore.  BMW probably just used approximations.   Although it was never intended for the R75/5 per one Airhead Guru, the fact remains that is IS LISTED in the official BMW Snabb Katalog for ALL the /5 models, & isn't shown for the /6 models at all; ....THEY used the -650 gasket, which did less lowering of CR.  I get into this a wee bit more, two paragraphs downwards.

The 11-11-1-257-092 gasket/shim, mentioned above, is, as noted, sometimes identified in BMW literature as "96ROZ", but that identification is also used for the 11-11-1-335-650 gasket noted a bit later here.   It refers to the octane of the gasoline, but not clearly, & is not directly equivalent, even with a correction factor, to the USA method of gasoline ratings (R + M divided 2).  Suffice it to say that the gaskets reduce the compression ratio some, compared to non-use, so that a lesser grade than premium fuel may be used.  When the compression ratio is lowered, power output suffers.

The 11-11-1-257-092 gasket/shim was specified for the R50/5, R60/5 and the R75/5, when it was desired.  This is shown clearly in the early fiche, & in the Snabb Katalog, & especially in Euro bulletins from the factory.   The CR reduction is not the same for all these models, when that gasket/shim is used.  The REASON for that is due to piston size (you can refer to a compression ratio calculation formula if you are nerdy enough).  For the R60 series, the reduction is 9.2 to 8.6 in some books, but is actually a reduction to ~8.2; and for the R75 it supposedly reduces the original 9.0 used on some, to 8.5, but that is in error, & the reduction is a bit more, closer to 8.2.

For ~1976 to ~May 1979, TWO SHIMS were available,....the so-called Step 1 compression lowering gasket was .7 mm (.028"); and the very low compression gasket was 1.2 mm (.047").    A point of contention is whether or not these were STEEL or aluminum, but they had 99 mm central holes, and are thus easily distinguishable from the earlier 97 mm aluminum ones. 

>>>>In late 1975, BMW lengthened the cylinders by 0.5 mm, and eliminated the gasket, & increased the spigot from 97 mm to 99 mm in diameter.  The cylinder top studs area used O-rings, the cases being machined for them.  NO standard cylinder base gasket/shim was used!

The compression lowering gasket that was AVAILABLE was 11-11-1-335-650.   Aluminum, 0.7 mm (0.028"), reduces compression from 9.2 to 8.6.  This is also in some literature as "96ROZ", and note that this gasket is in the literature as available for use in ALL models of /5 and /6.  So, there is more than just one compression lowering plate that is called "96ROZ".  WAS NOT used on new bikes, it was just AVAILABLE.   It is all rather a bit confusing, due to the description, & that the compression lowering also varied.  It was used, supposedly, after 09/1975.   NOTE that this gasket has the 99 mm hole so it fits the later bikes....what about earlier?....Sure!

Note that this gasket/shim can be used in place of the 11-11-1-257-092, but this one is less thick.  In the Euro bulletins, this gasket was to be used to reduce pinging in the R60 models (/5, /6, and /7).  It was to be used with a timing setting of 6 BTDC (before OT), and highest carburetor needle setting (maximum richness).

BMW confuses itself about the reduction in CR. BMW further confuses itself in the bulletin in which all this is described, by specifying it is used with the 11-11-1-262-141 Cord Ring Seal (that means the TWO small rubber O-ring at the top studs at the engine case area).  The gasket can be used for earlier engines, that did not come with those O-rings. 

NO large O-ring groove was used on the cylinder you do NOT add an O-ring!  NEVER add the large O-ring, unless the cylinder is grooved for it!...or you know what you are doing about groove modifications!

BMW confused itself even more with some of the descriptions of the -092 and -650 gasket.  This started with the Service Bulletin, issued in Europe, about these gaskets.  BMW said that both of these gaskets reduced the R60 series bike's compression ratio from 9.2 to 8.6.  That is not possible with TWO thicknesses!  In actuality, the 1.2 gasket (-092) reduced the CR by one full point.

While the R75/5 came with gaskets (obsolete number 11-11-254-084 if I recall correctly), that does NOT mean all the /6 models came with gaskets. 

One of these Service Bulletin's, if you are curious, & this is the one of interest, is dated 2/73 (347).  The Service Bulletin identification numbers ARE correct.

BMW changed the sizes of pushrod seals & tubes.  Information in my article on pushrod tubes.

You CAN machine a cylinder for a base large O-ring.  While there is no big need, you might be interested, and, you may want to look at a later engine case OPENING, see what that looks like (especially the opening chamfer).  It is entirely possible for the O-ring and chamfer changes to not exist at the very same time for some engines. 

For 05/1979 to 1980 models, BMW added the O-ring groove at the cylinder spigots, obviously so an O-ring could be used.   The original O-ring was 11-11-1-335-866, which was WHITE, 93 x 2.0 mm.  It is no longer available.  The replacement is 11-11-1-337-567, and is 93 x 2.2 mm.  I suggest OILING IT JUST BEFORE INSTALLATION OF THE CYLINDER....certainly within ONE hour. 

Keep in mind that BMW makes next calendar year bikes at the end of the prior year, beginning just after the Annual August Vacation.  It is also likely that NOT ALL 1978 manufactured as 1979 models were so-machined for the large O-rings.  There have been reports of some 1978 models having the machining. 
 It is potentially possible that the 2.2 mm large O-ring might not fit a few very early cylinders, but the 2.0 mm O-ring is supposedly no longer available.   This has not been clarified, I have received conflicting information on what BMW has shipped at times.   The cylinder O-ring area appears to be:  OD 3.8899"  98.78 mm; groove depth is .024" or .61 mm; groove width .104" or 2.64 mm.   BMW issued a bulletin long ago, that said it would be using the 2.0 mm O-ring ONLY during production, & if weeping was seen, a dealership could change the O-ring from the 2.0 to the 2.2 mm.  Note that later cylinders have slight changes besides just having the large O-ring, and if retrofitting, be sure to modify the cylinder bases as required.

Clarification (I am sticking this note HERE, but it applies to this entire article):
Earliest engines do not have counter-boring at the top studs, at the engine block, because O-rings were not used then.  Next came the counter-boring in the CASES, for those small O-rings.  Slightly later the cylinders themselves got the machined groove for the large O-ring.  Machining is possible, for cylinders & if warranted, the cases, for whatever combination of cylinders & cases you want to use.  The later cylinders not only have that machined groove, but a slightly different shape near that groove which affects sealing if the case does not match, & the LENGTH is also different, which causes a difference in Compression Ratio, by about half a point. You can install ANY cylinder that fits the case hole, with OR without the large O-ring, with minor cylinder machining if and as required.  If the cylinder spigot is too large, the case needs to be machined too...or, you can machine the cylinder.

IMO:  In EVERY combination of cases & cylinders, you SHOULD use a sealant, even on those cases & cylinders that use the small & large O-rings.


Because of the annoying amount of labor to fix a tiny leak, & the safety of Hylomar, in the past I ALWAYS used Hylomar SQ-32L, on all Airheads... even on the later cylinders with the big O-ring at the base in the groove.  I never had leaks, & I oiled the O-rings within minutes prior to reassembly. I still use Hylomar myself, although I may use other sealants now and then.  When I have no Hylomar left, I am not sure if I will purchase it again. I might use a newer sealant.  I no longer recommend you use Hylomar. Hylomar can fail if not done near perfectly.  Use of later types of sealants is approved by me.  Please see my CHEMICALS article for sealant recommendations.

If your bike uses a shim (sometimes called gaskets) of metal between cylinder and engine case, you can coat all 4 surfaces VERY thinly with your sealant, but you can also consider just coating both sides of the shim.  Absolutely keep excess sealant from the upper stud areas.

Hylomar is generally quite safe to use, although slopping on a mess of the stuff, like ANY sealant, is BAD!  Only the thinnest coating of a sealant should be used & the surfaces must be VERY clean.  It is important to have the sealant around the outside of the studs holes, but be careful about using too much at the top studs where the oil delivery holes are. It is important that the CONROD not be allowed to fall onto the case opening, WILL cause a nick that can prevent proper sealing & oil may weep out.  If you get a nick, dress it out!   If you ARE neat & careful, there is no big reason NOT to use Hylomar.  If you use a brush (I use my fingertip) you must NOT leave any bristles!

Folks ordering the -866 O-ring may well get the -567, even if they tell the dealer parts department person that they want only the 2.0 mm.  

The 1978 and earlier WITHOUT the large O-ring MUST have a sealant used, but it is OK to use sealant (& I do) on the later models. Be careful with sealant around the top studs (oil passageway area).   I use it very sparingly & as evenly as I can with my fingertip, & around the outside of the studs area on the block...NEVER on the studs themselves. NOT into the oil passageways!!  Remember that the cylinder base will tend to squeeze almost all of this sealant out...and 'out' can mean into the oil passageways.  

A decent sealant is expensive Dreibond, or Three-Bond from your BMW dealer.  The following are tested & acceptable:
Permatex 27 Hi Temp RTV
Pro-Seal Red 700 degree RTV
The Suzuki sealant is fine too. 
There is a Yamaha sealant too.   for the latest information!
Be SURE to allow the sealant to 'set up' for half an hour (less on some types) BEFORE assembling.  THUS, the order of events is sealant, wait, oil the O-rings..... immediately assemble, torque properly. 


Removing cylinder shims.
Ridge removal.
Measuring cylinder taper.

Damage has been done to cylinders by improper using of ridge reamers; & the damage is relatively easy to do.  You can purchase a usable ridge reamer for less than $40 at an auto-parts store, but I am not suggesting you do, or do not.  Just be aware that there are some things the makers....and well-meaning NOT tell you.

First...what is the ridge, and need for a ridge-reamer, anyway..???
The ridge is an area of hard combustion products and/or NON-wear.

If you remove a base shim, the piston will go further up the cylinder (the lower area is a problem too, see later). Classic engine work texts, etc., say you must remove any ridge.  There is a lot more to all this, besides the fact that you might NOT have any ridge. If a piston ring smacks the ridge, the ring could break and/or piston damage is possible.

The main purpose of a ridge reamer tool is to allow the piston, with its rings intact on the piston, to be pulled out of the cylinder in the direction of the head, without the rings 'catching' or 'hanging up' on the ridge. The process is very simple, one rotates the engine so the piston is down the cylinder, and then uses the ridge reaming tool at the top area of the cylinder.

There are two basic types of ridges.  Due to wear, there may be a stepped ridge, beginning as far down the cylinder where the bottom ring (usually called the oil ring) stops when the piston is fully down the cylinder.  This would happen more on old style cylinders rather than Nikasil types which hardly wear at all.  The other type of ridge is typically more pronounced, is ABOVE where the top ring stops in the cylinder, and occurs due to some basic effects, which generally are more and more pronounced with accumulated miles, and oil-burning engines tend to accumulate such a ridge at a faster pace.  The effects of the many miles/hours of combustion products are deposits, etching and re-supplying of metallic deposits, and similar fun and games all tending to leave a ridge, and the ridge can be rather hard. In the old leaded-gasoline days the top ridge was usually considerable on high mileage vehicles. The over-all effects are an upper ridge that makes the cylinder diameter smaller at the ridge point. Sometimes this ridge is seen as discoloration, but can not be felt by your finger or fingernail, certainly that is common for the lower ridge. In other instances, I have seen rather nasty top ridges, particularly on cars, that might be .025"  to .040". Airheads usually look pretty good.  Removing the piston through any 'ridge' area might damage the rings.  

Besides the top of the cylinder area, there can be a problem at the place on the cylinder that the oil control ring (the bottom ring) visits.  At Bottom Dead Center (BDC) the piston has traveled as far down the cylinder as it can.  When you remove a shim or gasket or otherwise have the cylinder closer to the engine block, the rings are in a new position, ever so slightly.  Old Airhead cylinders tend to have taper wear, & you want to be sure that the rings travel over very nicely round NOT tapered areas.  This can be done with careful honing (if boring is done, proper use of pressure plates & boring equipment will remove cylinder taper).

Removing a piston from the top of the cylinder is seldom done in Airhead work, as usually there is no reason to do so. On almost all cars it is different, the cylinders are not individually removable. And, if the situation was such, you'd hardly want to remove the crankshaft to get a piston out.  On an Airhead, when you pull off a cylinder, the piston goes out the cylinder in the bottom direction, & any top ridge is not then involved; but there might be a bottom area that is smaller in diameter, just below where the oil ring stopping point. Usually there is no problem, & the Airhead piston with rings removes easily.  PROTECT the rod & engine NOT allow the rod to fall, piston attached or not.  If you do allow it to fall...the case will be nicked, & you will have to dress (fix) the case, otherwise you could have oil leaks.

If the ridge reamer is used improperly, & that is easy to do, you will INcrease the bore size in that area, and if you THEN planned on honing and certain other work, you might have to hone even further than you otherwise would! This can make for oversize situations that need to be dealt with.

Where that ridge IS especially involved is if you change pistons to another type, or brand, or size. The top & bottom rings may not be in exactly the same position compared to the old piston.  You might also be removing a shim from the cylinder base, which may reposition the rings.  Another reason might be if you are machining the base of the cylinder to raise the compression ratio...this is sometimes done with or without machining the head.

I usually carefully remove the ridge, if there is any, by hand; using a ridge reamer if appropriate. In some instances just hand scraping will do, I have a hard plastic & hard wood scraper, besides a very old-fashioned triangular metal scraper designed for scraping bearings, ridges, etc.  If the cylinder was going to be bored, to fix damage or taper-wear, etc., the ridge is automatically removed by the machine shop & the subsequent honing finishes off the cylinder, so nothing to be concerned about.

I advise against using a ridge reamer, unless the ridge is really bad; or, you are changing something that would put the top piston ring into the ridge area. This is especially so if the ridge cannot be felt by a fingernail. Quite a bit of a ridge can often be removed by chemical means. I'd be tempted  to have the cylinder upside-down in a glass cooking dish, with an in or so of 50% Simple Green, overnight, then scrape with a hard wood scraper.

If removing rings, know that they are exceedingly brittle.   I advise very careful removal from the piston. Using a VERY thin feeler gauge to help slide the ring off the piston works very well, & helps avoid breakage.

Do NOT mix up the rings between the pistons & do NOT put rings in the wrong place or upside down from what they were. Do one cylinder at a time. 

I am very anal about this, and NEVER trust old rings to have been installed correctly; and am VERY careful to install new rings carefully.  The new rings might be a different type, so pay attention to manufacturer's info!


Bore wear measurement, bore taper measurement.
New rings fitting, etc:

Measuring bore diameter & taper wear & measuring rings wear or to adjust new ring end gaps, can all be done without needing tricky-to-use inside micrometers.  Measuring bore diameter needs to be done VERY precisely. Few except professional mechanics own the proper type of inside micrometers, certainly I'd not expect anyone to own the automatic air operated industrial type; & the old-fashioned mechanics' type requires the use of an inside ball or rounded noses type of micrometer (there are also blade transfer types & then one needs an outside standard anvil micrometer to READ the inside type). Certainly, you could take your cylinders to someone with the instruments to do accurate measuring, a good idea, actually!

I own a set of REAL inside micrometers meant for cylinders, they have a readout scale...but I don't expect you to, nor, anyone except maybe a pro-wrench.   MANY a person has used inside micrometers & gotten wrong readings due to technique.  It is EASIER to NOT make such a mistake, with my below method!

So, what to do, besides taking your cylinders (pistons need measuring for wear too, so take them along) to someone?  A really good method exists, it just takes a bit longer to do. It is also ACCURATE.  My method for you is to use a RING & your PISTON....& feeler gauges. You must be careful not to break the ring, but this method works, & really is quite accurate.  I used to keep old worn used rings on hand for these purposes; but the ones off the piston in use can be used quite successfully.

1. Remove all the rings from a piston, noting tops, bottoms, markings, etc.  Use my THIN feeler gauge method of easing the rings off the piston to avoid breakage.  Prior to doing any of this work, perhaps you should read this:

2. Insert the top ring, from the bottom, and reasonably squarely;...careful!...don't break it! Use the ringless piston to push the ring a bit from the bottom of the cylinder, & the piston will square-up the ring in the cylinder bore.

3.  Now use one or two-together, as required of your feeler gauges, to measure the ring end gap. Write it down.  Move the ring further up the cylinder, perhaps into the half-way area, re-measure. Do it again at maybe 1/4" or so below any ridge or ridge discoloration area.  Re-measure. The ring gap can be mathematically converted to diameter differences, thus you know the TAPER of the bore which can be expressed as a diameter or differences in diameters.  Remove the ring to the bottom (or top if no ridge). Be sure to be careful doing this, keeping the ring square, etc.

4.  If the taper is acceptable, carefully reinstall the rings to the piston, if the ring end gaps are reasonable. AGAIN be careful about not breaking the rings. BMW has specifications for end gaps, it various with year/model/ and which ring....but, basically, you can go beyond the specification, larger gap that is, for the rings. SPECIFIC Hints are in the LINKED article, ABOVE. I've seen rings, pre-Nikasil cylinders, that were hardly worn after lots of miles...and I've seen some that were quite bad. Wear comes from many places, besides just mileage, including the use of BAD air cleaners like the K & N. I am not about to get into any arguments about K & N here....there is an article on them, here:

5. Pistons can be measured in the bore too, but you won't have the thinnest required gauges.




Stud centers are 93 mm x 72 mm.

01/26/2008:  new
06/22/2009:  clarifications, minor only.
03/06/2011:  updates, clarifications, move some cylinder recommended places to where it should be, in the REFERENCES article.
01/07/2012:  Clarify the shims information
09/24/2012:  Add QR code; add language button; change Google code
11/20/2012:  minor changes and link regarding sealants
2013:           remove troublesome language script code
07/25/2014:  Clarify 96ROZ for both -092 and -650 gaskets; & that both of these gaskets fit /5 & /6.  Minor other clarity fixes, such as use of 'gasket' which includes the shims in BMW literature.  Added red note on ridge removal, which is noted elsewhere's on this site.
09/28/2014:  Clean up article; again improve clarity.  Add KS logo jpg.
10/06/2014:  Add entire section on shim removal, ridge removal, measuring cylinders.
12/08/2014:  Add/modify note on combinations of cylinders, cases, etc....for clarity.
09/28/2015:  Clarify a few details
02/07/2016:  Narrow article; increase font size; update meta-codes; clarity of article improvements.
05/26/2016:  Metacodes, improved layout, remove many redundancies, check scripts, increase section separations, final updates.


Copyright, 2013, R. Fleischer

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Last check/edit: Friday, June 03, 2016