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Pistons (including swapping heads-cylinders-pistons);
all about piston rings, pins, clips, keepers, de-carboning, ETC.

FOR:  BMW Airhead Motorcycles
Copyright, 2013, R. Fleischer

pistons.htm
Article 60, sub-section 4


Be sure to read this article:  cylinders.htm


De-carboning:
Decarboning is done for various reasons, but especially to keep compression ratio from increasing, & for keeping hot-spots out of the cylinder head.  Decarboning of the top end, heads, pistons, etc. on a BMW Airhead Motorcycle is best done with the pistons & rings removed & separated from each other, & the head disassembled.  I suggest that you do NOT take the top end down that far just to de-carbon it.  Leave the valves and springs in the head.  Removing the head & cylinder enables the head to be soaked in something like Gunk (brand) Hydroseal (up to a week!); or Simple Green (50%, overnight).
 
It is easy to break the VERY brittle piston rings.   That is why I usually recommend NOT removing the rings from the piston.    The piston (with rings still in the piston grooves) can be removed from its rod, and similarly soaked, rotating the rings now & then.   Removing the piston pin rearwards, after removing the rear piston pin clip, is usually fairly easy, if the piston is heated with a hair-dryer to expand the aluminum (much faster expansion than the steel pin).  There are both inside & outside piston clips in use, if you have a 1977, it is a bit of fun. 
DO NOT scratch the pin bore area in the slightest.  If need-be, use a draw-bar method if the pin is too tight, even with the piston quite hot.  DO NOT let the rod fall & nick the crankcase!  There is a correct, & incorrect, inwards side to some clips.  Do not mess up the piston circlip if you intend to reuse it.    I replace them every time.  When removing or installing clips, do not do more than just the needed amount of pressure & movement.  

>>>>Wrist pin circlips MAY have a sharp side and a rounded side; if so, the rounded side goes INWARDS.


HINT:  Some will leave the pistons/rings/rod-to-crank all intact, just removing the head and cylinder, and use a heavily soaked rag on the piston, with the liquid being Simple Green.  This DOES WORK.

DO NOT glass-bead blast the heads, nor cylinders, nor pistons.  Hard media will get into everything, & even after very careful cleaning some is left, even into the metal surfaces and may get into all sorts of other areas, like bearings, and upper end areas where they will cause fast wear.  Besides those problems, blasting pistons with glass beads can easily be overdone, can cause WEAR just from the blasting, let alone the residues that did not wash/brush out.     DO NOT use such blasting media.    Walnut shells can be used.  Use of soda or especially CO2 blasting, is fine....the soda method requires a good cleaning afterwards.


Swapping heads, cylinders, pistons:

MANY questions have arisen on swapping cylinder heads, cylinders & pistons between the various models of Airheads.  I am NOT going to discuss every possibility.   If your question is not answered HERE, ask on the Airheads LIST.

        a.  In 1976 BMW changed the crankcase to accommodate the 99 mm base cylinders, previously 97
             mm. Thus later cylinders don't generally fit earlier cases; but can be machined.  Some early 99
             mm base cylinders were made without grooves for the large O-ring, all later ones had that very
             large O-ring, having a machined groove for it on the cylinder base. It is possible to install Nikasil
             cylinders on some pre-1981 engines.  You have to trim a small amount of material at the bottom
             of the cylinder, inboard of the O-ring groove. You need the cylinder in a lathe to do this. 

             This needs explanation, as there are two styles of Nikasil cylinders. The first versions, used
             between 1981 and 1984, were the same as the earlier cylinders, that is, they do not have a small
             step above the O-ring groove, & fit the earlier cases directly with no problems. LATER than 1984
             cylinders do NOT directly fit the pre-1981 cases......... you have to machine-off the step.  YOU
             COULD USE WITHOUT THE O-RING. 

        b.  You can fit Nikasil cylinders pistons from a R80 in a R75/7.   The squish band is different on R80
             pistons, but the clearance is OK.  Problems are with the R100 engines with HIGH
             COMPRESSION pistons.
        c.  1000 cc cylinders won't fit R90S heads.  There is a head gasket difference with the 1000 & all the
             smaller displacement sizes.    You COULD use 1000 heads and cylinders and pistons.
        d.  The high compression heads & pistons were on U.S. 77-78-79 models of the R100.
        e.  It is possible to combine European high compression ratio pistons & heads & Nikasil cylinders.
        f.   Almost always, using large displacement on a given engine that came with smaller displacement,
             will result in more vibration...or power pulse feeling.
        g.   Do not mess up the piston circlip if you intend to reuse it....I do not, I replace them every
              time.  When removing or installing circlips, do not do more than just the amount of
              pressure & movement needed.   The wrist pin circlips have a sharp side & a rounded side.
              The rounded side goes INWARDS.   
Old style pistons with internal pin clips interchange with
              later external clip types.   Airhead pistons up through EARLY 1977 used a thin circular INside
              wire piston pin clip.  The later 1977 & perhaps some very early 1978 had a THICKER version of
              that clip. The piston pin clip on most if not all 1978 and later had the EXterior type.
       h.   High compression pistons may have problems with the head.  Later pistons had the dome
             extending to the edge; later 1970's types with the squish band were designed to work with a
             matching cylinder head, & might have to be machined.   This might occur with some modification
             you are attempting, such as later pistons fitting 1977 heads.   The squish pistons used on such as
             the 1977 R100RS are hard to find.  The original 'squish' pistons are no longer available.  One can
             modify the head & cylinder (deck height change, about 0.050")  and thereby get your own
             squish pistons in practical use.   The 1977 year was different regarding the pistons.    The
             1978+ had a different shape for the pistons...at the edge as an example.  The 1977 style pistons
             are probably no longer available.  Perhaps some PRE-emissions 1978 pistons will work, but you
             would have to order them & look at them.  The 1977 R100 pistons were special, only used in that
             year & model.  If you have a 1977 R100 & need pistons, you could consider using the 9.5:1  1978
             pistons with the outside circlip.
        i.   You can NOT trust the latest parts information at the dealerships.
        j.   To remove the inside type of piston pin clips, use a hardened tip sharp tool with an L shaped tip.
             Insert the L shaped tip into the tiny recess, getting the tool behind the clip; move it towards the
             center of the grooved hole, so as to remove it. 
        k.  To remove the OUTSIDE type of clip, use a conventional circlip pliers.  DO NOT mess up the
             groove!  The SLIGHTEST burr on the groove MUST be fixed, otherwise the pin will not fit
             smoothly.
       


Piston ring rotation; orientation; cylinder crosshatch:

Piston rings DO rotate. 
They are designed to rotate; only a few engine types do not have rotating rings.  Those non-rotating ring engines are mostly the 2-stroke type with ports on the cylinder walls, that you would not want the ring ends to grab onto or be opposite.   There ARE some 4 stroke exceptions.    The roundness of the cylinder has an effect, and there are some smaller effects.   The cross-hatch pattern that is done by the factory is very specifically designed to cause ring rotation during break-in.   When boring or otherwise renewing a cylinder when real honing is being done, the crosshatch has to be done correctly.   The proper cross-hatch is done with a specific angle, best done on an automatic machine, and if done correctly it will hold a miniscule amount of oil during break-in, and cause proper ring rotation at that time.
My cylinders article gets into this deeply.

Rings should be set roughly 120 degrees from each other 'IF' you removed the piston assembly from the cylinder. I HIGHLY disagree with those that say it does not matter if the rings are lined up any which way.   Rings do move but mostly with fresh crosshatched bores and initial few miles of break-in, then they tend to be stable; however, they can move (rotate) again; and likely will, particularly on the Nikasil cylinders.   You should not put the oil ring end gap at the bottom. The reason is an accumulation of oil...particularly when the bike is on the side-stand. 

Note that genuine BMW rings usually have a top & bottom, with the top marked, sometimes very faintly.   Due to wear patterns, besides ring design, ALWAYS return the rings to the same top/bottom orientation. Do not install a ring upside down from a specified orientation in the first place.  BMW piston rings are usually marked TOP near the ring gap.  Don't assume that pistons, like new ones from a dealer, have the rings on correctly.  The older style of oil rings may or may not be marked TOP.  Failure to observe the orientation of the ring will result in high oil usage. If put in upside down, very easy to do, your cylinder will burn oil.

The TOP RING, also called the compression ring, means the ring closest to the top of the piston.  Use a magnifying glass.   The inner portion of the ring has a tapered edge that faces the head (or top of the piston), that is, outwards. 

The 2nd ring (ring in middle) has a notched outer edge.   The notch faces the engine case.  On the 1981+ style 2nd ring, that ring is marked for the top, the bevel does face the crankcase.

From the earliest /5 to about 1980, the bevel of the OIL ring was towards the valve cover.   Old style oil rings have a taper on one edge, it is not easy to see sometimes.  If you have that type of ring, that taper goes OUTward, towards the piston top.  The wider rim of ring skirt was towards the crankcase...that is, the wider part of the taper was towards the crankcase.    This has caused confusion with the U channel ring.   There are two chamfers on that ring.  The chamfer on the EDGE faces the top of the piston.  The chamber on the INSIDE of the U faces the engine.    From 1981, a new, symmetrical design was used, there is NO UP NO DOWN! (for the OIL ring, that is), and there is a radial spring;  Nikasil cylinder bikes oil rings will have a spring expander behind it.  Early oil rings had a sub-groove, to allow gases to force the ring against the sleeve. 

Pay attention from which groove the rings came, and don't break them, they are VERY brittle.  Removing the rings is a job to be done very carefully, avoiding expanding the ring diameter, use the least possible amount.    Rings are very brittle, and slipping them off a piston has caused folks some problems...use a few THIN feeler gauges to help ease them off. Do NOT put much pressure on the rings, they WILL break.
Because of these regards, I sometimes advise folks that are not installing new rings, to leave the existing rings where they are, ON THE PISTON.  You CAN clean the piston and rings that way, with rotation of the rings, during the cleaning process.
 


ASSEMBLY NOTES:

LIGHTLY, EVER SO LIGHTLY, OIL the piston & rings when re-assembling; this is NOT the place to drip oil or use the oil can.  A bit of oil on your hands is all that is needed. 

Here is the proper orientation of the ring end gaps, based on the common analog clock, as you face that side of the engine:

LEFT SIDE: outer, compression ring: 4:30
2nd ring: 10:30
innermost (oil ring): 1:30

RIGHT SIDE: outer, compression ring: 7:30
2nd ring: 1:30
innermost (oil ring): 10:30


Pistons have a front/rear and you can cause SERIOUS problems if you do not assemble the piston to the rod with the small stamped arrow, located on the top of the piston near the center, pointing forward. There will be the word VORN (German for FRONT) next to the arrow. It may be covered with carbon.   If using aftermarket pistons, they may not be marked, and the valve cutouts will usually give you the correct installation, just from inspecting the cutout sizes, the intake being bigger.

DO NOT put the pistons that were in your left side, into the right side, & vice-versa.  Pistons operate such that one side gets much more pressure (thrust effects) than the other, wear patterns must be maintained, as well as shape.

Most Airheads are specified at APPROXIMATELY a maximum cylinder ovality of 0.0004" and a maximum cylinder taper of 0.0008".  The piston clearance is typically 0.001" on the smaller bores, to 0.002" for R100 engines.  The Nikasil/Galnikal cylinders use a piston clearance of approximately 0.0015".  

The factory specified ring end gaps, prior to Nikasil/Galnikal, were top ring at 0.012-0.020"; Nikasil/Galnikal use about 0.020" for the top and second ring; and 0.013" approximately for the scraper ring.  These values I give here are in brief, the factory manuals give the exact gaps for each ring for all the various models.  The information here, however, is OK.     What do I do, myself?   I don't go by the factory books, nor Haynes, nor Clymers, etc.    I fit rings for end gaps that are about 0.0035" per inch of cylinder bore.   DO NOT make the mistake of fitting rings with too little end gap!

 Note that BMW rings as supplied may have larger end gaps than the manual states; and are OK.  I would NOT be inclined to try to install rings to as tight as the factory specification....somewhat more end gap is actually preferable on our Airheads.   NOTE that one can use oversize rings to fit worn bores on the PRE-Nikasil cylinders and then adjust the ring gap by filing the ring ends.   This may not be perfection, but in practical usage it works fine, but do this ONLY on the first oversize (0.25mm).    DO NOT do that with the Nikasil/Galnikal cylinders.   Rings for Nikasil and Galnikal are NOT made the same way nor are they the same material as for the old steel cylinders!   Pistons seem to be the same for Nikasil and steel bore cylinders.  NOTE AGAIN:  The rings are NOT the same material and design.
 

There are plenty of styles of rings. 
  

Below is information on Pre-Nikasil piston and rings. The Nikasil rings have a spring expander, and the two bevels are opposed. 
Photos show order of installation:

         

 

Here are two additional photos, note the type of ring construction on this particular set of rings:

   



/2 era cylinders are not made the same way as the /5 (through all except the Nikasil/Galnikal and later).  The /5 and later, prior to Nikasil/Galnikal,
had cast-in steel liners. 

 
NOTE how the oil ring bevels are different on this type of oil ring, from the Clymers manual sketch type of symmetrical ring.

Note that some types of rings use a ring-expander behind the oil ring.  Nikasil rings for example.

Oversize pistons from BMW are NOT available for the 1981+ cylinders.  Selected graded pistons to fit the bore are available.   AFTERMARKET pistons ARE available.  

The following information on AFTERMARKET pistons is believed accurate, but has not been checked in great detail by me:
San Jose BMW (CC products) sold some pistons for a 1050 cc kit that gives 1038 cc.  These pistons were forged types, by Venolia.  The diameter was 3.810" (96.774 mm).  Alloy 2618, a low silicon, was used.  This means these pistons expand at a goodly rate when they get hot, so MORE piston clearance is needed.   Try 0.0035" minimum.   Alloy 4032 would wear better, and expand less, but 2618 is stronger.   If a crack occurs in 2618, the crack may well find an area of lower stress and stop....not so the 4032.  So, 2618 is a GOOD choice for piston material, and holds its shape better under high stress use too.

If you are going to use aftermarket pistons, ASK about proper clearance specifications.   This is a good idea for ring gaps too.

NOTE:   Various 'books' (including BMW's own literature) has somewhat conflicting information on ring end-gaps.   It is also MY belief that most of BMW's literature has the end gaps TOO TIGHT.    I prefer somewhat wider end-gaps.   I usually use around .5-.6 mm for all three rings.   Whether or not to change rings on an engine that is apart, yet is not an oil burner, is a good question.   This is particularly so on the Nikasil cylinders.  I would let them go until quite wide and actually showing signs of real oil burning.

Mahle makes a big bore kit using 97 mm pistons, giving 1043 cc.

Pistons are purposely tapered to be bigger at the skirt end, so be careful when measuring things.
See the following websites:
www.jepistons.com/pdf/piston_instrc2618.pdf
http://www.sjbmw.com/
www.venolia.com/main.html
 

You can also check with German websites, etc.,....pistons/cylinders/rings are available from Europe that are not available in the USA.

PISTONS AND RINGS ARE TO BE INSTALLED ""LIGHTLY"" OILED...not dry.  Lightly means the faintest amount of oil that can be used as an oily film, it does NOT mean squirting with an oil can!   A few drops, ON YOUR FINGERS/HANDS, then spread with those fingers.  Perhaps a FAINTLY oily rag on the cylinder walls.  I use a straight mineral oil.

If a groove that the piston rings fits into is heavily worn, the ring will rock with respect to the piston far more than needed, and things can get bad enough to break rings and pistons. Pistons DO wear out. 
 


MISCL:


You do NOT need an expensive tool to measure cylinder taper.  You might not bother to measure the taper on a Nikasil cylinder. For the earlier cylinders or Nikasil cylinders, why not do it...and do it easily!    Install a piston ring, squarely to the bore (use a piston, if you need to, to get square orientation), it need not be an original ring off your piston, just one that fits.    Use your feeler gauge set to measure the ring end gap, and do this at a point just inboard a bit from the top of the cylinder ridge wear area. Write down the feeler gauge thickness.  Redo the measurement near the bottom.   For each measurement, divide eby pi.   Compare them.   Simple, wasn't it!

Piston clearances generally, unless there is real damage, don't go much over a few thousandths...certainly not the 4.5 specification limit.   It is perfectly OK to use next oversize rings, adjusting the end gaps, when you have a bike with a good bore.   If you do this, we shall assume the bore has low taper, but the rings are perhaps on the edge of specifications or bit over.  You can purchase an oversize set of rings and adjust the end gaps to specifications, but when doing this, use the smallest measured part of the bore....usually near bottom, where the diameter of the bore in the lowest area that the rings normally contact, is the smallest.....adjust the new oversize ring gaps for that diameter.   

Cylinder base shim/gasket/compression plates:  see my cylinders article, that treats this in depth.


rev.:

released 01/26/2008
01/11/2009:  updated, mainly to clarify some details, such as piston pin clips
06/15/2009:  clarified a few details.
08/29/2011:  clarified ring orientation, as folks were NOT reading all the information.
08/31/2011:  More ring clarifications, photos.
09/05/2012:  Add QR code; add language button; change Google ad code, minor other.
06/13/2013:  Clarify some details on mixing up pistons and cylinders and heads. Go through whole article
                    with minor clarifications and clean up sections, etc.
09/28/2014:  With the changes made to my cylinders article, I updated the pistons article.
10/06/2014:  Add admonition to read the cylinders article.

Copyright, 2013, R. Fleischer

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