This page applies to the models of BMW that have a wristpin bushing in the
end of the rod. That would include the entire /3, /2, /5, /6, /7 and
The wristpin bushing for the /3 and /2 BMW motorcycle
The rod(s) of all BMW's made after the war have a bushing pressed into the
small end of the rod. Over time, with the stress of the pushing and pulling on
the rod, the actual hole in the rod will become elongated. It is common for the
hole to be .002" out of round. I have seen it be far larger due to an accident,
but I would never trust that rod again. It is also common for the rod to be a
bit bent. By using two parallel bars between the wristpin and the case, one can
see the bend. By moving the rod up or down and performing the same test, one can
often see a twist in the rod. The twist isn't important at all. If the bushing
is bored evenly with the hole in the rod, then the piston will be cocked to one
side. That is not good.
It is common to find the BMW motorcycle wristpin loose in the bushing, or the
bushing loose in the rod. The subject of replacing the bushing is one of some
misunderstanding. One rebuilder presses in a new bushing and to size it he
pushes a ball bearing through the wrist pin. That is a very poor procedure
and here is why.
As a ball goes through the newly pressed in bushing, it will follow the path
of least resistance. The "new" hole may be perfectly sized and the pin may fit
it well. The customer will find that all seems well and be content. However, the
ball will follow the bushing hole that is already there. If the rod is bent, the
new hole will allow the wristpin to be out of alignment, same as the rod.
That crooked hole will allow the piston to be cocked in the cylinder and may
cause less than a smooth movement.
If the hole in the rod is more than a little bit "out of round", it should be
reamed out to get it back to round. Then, press in the new bushing. With the
crankshaft in the case, one must mount up a fixture that bolts to the case. The
fixture will hold the reamer parallel to the case. This is very important, as
the cylinder mounts to the case, that controls the piston travel path. With the
wristpin bushing bored parallel to the case, the rod can be slightly bent and it
matters none. In effect, the wristpin bushing may end up thicker on one side and
thinner on the other side. It matters none, as the wristpin is now parallel with
the case. It is not likely for a ball going through the bushing to be
parallel to anything.
All of this assumes that the case is still aligned. In some accidents, the
face where the cylinder mounts can become distorted and out of parallel with the
crankshaft. That must be corrected before the wristpin bushing is reamed.
Other issues may exist too.
It is impossible to accurately install and size a wristpin bushing on a used
or rebuilt crankshaft that is out of the case. It must be mounted in a case. A
"test case" can be used to make this a faster procedure. Take an old case that
has a good cylinder mounting surface and cut it in half vertically with a band
saw. The crank can now be quickly mounted up and the fixture bolted to the
case for reaming.
The /5 and later BMW motorcycle wristpin bushings
The same things apply to the later engine design. However, it is far less
likely that a bent rod will occur. The case is different enough that it will
take a different fixture to fit the larger base design. Otherwise it is
about the same procedure.
An owner that isn't interested in things being exact, may get away with using
a ball to size the wristpin bushing. An engine rebuild is so much work and
expense, why get cheap with any part of it? It is up to the informed owner to
make that decision. This page is only to inform.
If I get questions about this subject, I will take photos of a typical
fixture and set up for reaming the bushing and post them here. This page was
initially written up a long time ago and forgotten. A recent question on
the slash2 list reminded me and I loaded it onto my website.