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 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.
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
1971 R60/5 cylinder with a large "ring compressor" beveled
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
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
Ring gaps for the BMW motorcycle
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.
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.
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.
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,
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
More later when I get pictures of actually installing a cylinder. It is
so fast that I will have to use high speed film.