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BMW motorcycle piston ring compressor

By Duane Ausherman

The question has come up many times about what to use as a ring compressor when installing a cylinder on a BMW motorcycle.  All BMW shops, to my knowledge, do it the same way, with our fingernails.  It is faster and safer.  This has brought about a cry of protest that is huge. 

Upon reflection I now know that I was wrong.  It is very important to employ the special tool provided.  In the future I will always use the correct special tool.  Lets look at this issue. 

Car stuff

Car engines are what most people are used to and from them we gain a lot of our experience.  Most cars have a head(s) that are on top or an angle that still makes them nearly on top of the engine.  To install a piston one must have it already on the rod.  The whole thing gets shoved down into the bore and then the rod cap gets installed.  The cylinder wall is completely straight and this makes it hard to get the rings to go down into the cylinder.  They want to "catch" on the top of the block.  A tool is used to compress the rings so that the piston will easily slide into the cylinder.  The tool is usually referred to as a ring compressor.  It is a wide band that tightens up on the rings, the rod is inserted into the cylinder and the whole thing sort of sits on the edge of the ring compressor.  Then the piston crown is gently tapped down and it goes into the cylinder.  The ring compressor ends up loose on the top and has never even touched the piston skirt.  This works very well.  Sorry, but I can't find mine to photograph. 

BMW motorcycle stuff

With a BMW motorcycle the rod and piston are already installed on the crankshaft and the cylinder is slide over the piston.  It is completely backwards of the car procedure.  One could install the very same ring compressor and then tap the cylinder to onto the piston.  The cylinder base would shove the ring compressor down along the piston skirt.  Then one would reach in and disconnect or loosen the compressor and remove it.  By the way, the ring compressor must be the type that can be opened up completely so that it can be removed from around the rod. 

BMW was afraid of a ring compressor sliding down along the skirt to possibly damage it.  The ring compressor is also being shoved inwards against the piston by the "bevel" so that it is really dangerous to the piston.  I have several times seen gouges in the piston skirt from a ring compressor.  BMW was right, misuse of a tool can damage the piston.  To eliminate this possibility, BMW designed a ring compressor that wouldn't damage the piston.  Just so that it wouldn't get lost, they also attached it to the cylinder.  This clever invention isn't new at all.  I have seen it on every BMW cylinder that I have ever removed.  The oldest one was a R52 from 1927, so I can't really say that it was used from the start.  Now I wish I had removed a cylinder from my old 1921 M2B15 BMW engine. 

This shows the bevel at the bottom of the cylinder of a slash 2 BMW motorcycle.

R50 cylinder from the mid-late 50's. 

See the shadow that goes across the bottom of the cylinder base? It clearly shows the beveled edge.  That is the BMW special tool for compressing the rings.  Sorry, but that is the oldest one that is available to me for a photo. 

This shows the bevel at the bottom of the cylinder of a slash 5 BMW motorcycle.

1971 R60/5 cylinder with a large "ring compressor" beveled edge. 

This shows the bevel at the bottom of the cylinder of a slash 6 BMW motorcycle.

1974 R90/6 "ring compressor". 


 The typical car procedure

1.  Locate the ring compressor

2.  Tighten it up on the piston

3.  Hold the cylinder in one hand and tap the cylinder onto the piston.  Meanwhile the piston is trying to fall down on the loose rod.  This shoves the compressor down along the piston.  Oh, did I mention that this requires three hands? 

4.  Stop and balance the cylinder while loosening the compressor, spreading it out and removing it from the rod.   

5.  Examine the piston for damaged skirts. 

6.  Put your tools away. 

The BMW procedure

1.  Rotate all ring gaps up to the top. 

2.  Hold the cylinder up to the piston crown. 

3.  Lift the first ring up so that it starts in at the bottom and gently push the cylinder on with your knee.  Wiggle the cylinder slightly to encourage the bevel to "walk" the ring into the groove.  The cylinder will be slightly crooked on the piston. 

4.  Gently use your thumbnails to stuff the gap ends down into the groove.  The last part of the ring to go in, are the ends of the ring. 

5.  Repeat with the other rings. 


My memory told me that this was very quick, but 30 years has a way of clouding an issue.  I just happened to have a block with cylinders removed.  My first attempt took 58 seconds, but I was really clumsy.  My second try took 28 seconds and while I could improve a bit, that was good enough for me.  How long do you think the "car method" will take you? Are you willing to take the risk of damaging your (how much did they cost you?) very expensive pistons, or rings? We charged by the hour, how much "useless time" would you like to pay for?

Ring gaps for the BMW motorcycle

The Myth

Everybody "knows" that ring gaps must be placed away from each other.  The fear is that the compressed gases would leak out if the gaps are lined up.  Many suggest placing them at 120 degrees from each other to reduce leakage.  If you are using the BMW ring compressor, that makes it a bit slower to reach around and under the piston to shove those ring ends into the cylinder.  Let's think about this for a moment. 

The Reality

Each time the crank makes one revolution the piston also travels up and down one time.  For easy math let's use 6000 rpm.  During one second the piston makes 100 trips up and down.  True, only 50 are compression strokes, but for this discussion of time it matters none.  Half of the trip is downwards and there is no compression.  The half going up is now happening in 1/200th of a second.  That isn't very much time.  If all of the ring gaps line up, how much compression do you think is lost by gas going straight through them? Nothing worth considering. 

Now lets think about another issue.  The rings don't stay in one place.  Proof is twofold. 

1.  A two stroke engine has pins to locate the rings to prevent them from rotating and getting caught on the open ports. 

2.  Set your ring gaps anyplace you want.  Ride the bike 1000 miles and remove the cylinder.  You won't find them in the same place. 

You have three choices.

1.  You must accept that the rings rotate and just go riding.

2.  Redesign the pistons to locate the rings in one place.

3.  Remove the cylinders very often to put them back where you have decided that they belong. 

I think that the first one is the best one.  Forget about the ring gaps. 

The cylinder has a cutout to clear the crank web and it is curved.  Some like to put the ring gap there and use a tool to shove the ends in.  I never did it that way. 

This photo shows a slash 2 cylinder curved cutout for the crank.  It can be used to shove the ring gap into the cylinder.

See the curved cutout?

It is impossible for a car to use this bevel or curve idea.  The cylinder would have to have a bevel at the top of the cylinder.  I don't think that the combustion chamber people would like that idea much.  They are the ones that go around muttering about turbulence, flame front, squish band and a lot more high tech stuff. 

Big controversy

The usual reason for the cylinder to come off is to replace the pushrod donuts.  They get old and hard, then start leaking oil.  They can be replaced without removing the cylinder or piston.  If one wants to replace the cylinder base gasket, then the cylinder must come off.  That gasket can be cleaned up and reused many times. 

Different people suggest one of two ways.   Until recently I had never heard of anyone using this method.  The well known BMW tech person, snowbum suggests that one pull the cylinder back just far enough to get to the wrist pin and remove it.  That way the piston stays in the cylinder.  I find it far more time/work and a greater chance of error in that method.  One is also giving up the chance to examine the piston and rings.  Just pull the cylinder off and keep the piston on the rod. 

With the piston out in the open one can really take a look.  If the bike isn't burning oil, but has an unknown number of miles on it, I would want to look at the ring land (groove) wear.  Is the ring sloppy in the ring land? See the specs in the book.  Take a look at the upper 1/2" of the cylinder bore.  Can you see where the piston comes up to the top of its stroke? Does it show a ridge? Can you feel it with your fingernail? At the top part of the stroke the cylinder wears more than at the bottom.  This is known as cylinder taper.  A machine shop has a really neat tool to measure this wear.  If you can't feel a ridge with your fingernail, then I wouldn't worry about it.  It is possible for a cylinder to have 75,000 miles on it and still have a good bore with very little wear showing. 

Suppose that you want to measure it, but don't have the tools.  I will describe a very simple and accurate way.  Remove the upper ring.  Do it carefully as they are quite brittle.  I use my fingernails to open it up a bit and slip it off.  Stick it down into the bore just at the top part where the worst wear shows.  The ring will expand until it fills the bore at the largest wear spot.  Make sure that the ring isn't too high or too low in the cylinder.  Use a feeler gauge to check the end gap and record it.  Now slide the ring down to the bottom of the cylinder and check the gap again.  The difference is the wear times Pi.  Take the difference and divide by 3.1 to get the actual taper amount.  Now, wasn't that simple?

You can also see how much ring wear exists.  You really have no way of knowing what the ring gap was set to by the previous mechanic, but one can take a good guess.  The gap should be about .003" per inch of bore diameter.  For most BMW's that amounts to around .010." If you can stick a .025 feeler gauge in the gap, that ring is worn fairly badly.  If it wasn't burning oil, I would consider leaving it alone, but that depends upon your expectations.  Even it if was burning oil, maybe it was going past the valve guides and not the rings.  Oil going past the rings will leave the typical blue (white) smoke out of the exhaust. 

Test for burning oil

That test isn't hard to make, but most fail to do it correctly.  Get another person to ride or drive behind you at night.  Do this in a clear place on a calm night.  At around 40 mph in top gear, crank it on hard with full throttle.  Take it up to around 75-80 and then back off and let it coast down to 40 again.  The person behind will be seeing the smoke through its length and it will be more visible to him/her.  You may not be able to see it.  The smoke that you see on acceleration is due to rings and while backing off is valve guides.  It burns on both, then you need rings and a valve job.    

More later when I get pictures of actually installing a cylinder.  It is so fast that I will have to use high speed film. 

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This page was last edited: 04/14/2006 - copyright Duane Ausherman
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