The wrist pin bushing for the BMW motorcycle
This page will explain some of the perils one needs to know when working on anything related to the rod or wrist pin and bushing.
The rod(s) of all BMWs made after the war have a bushing pressed into the rod’s small end. Over time, with the stress of 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, and I would never trust that rod again. It is also common for the rod to be a bit bent. One can see the bend by using two parallel bars between the wrist pin and the case. 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 wrist pin loose in the bushing or the bushing loose in the rod. The subject of replacing the bushing is one of some misunderstandings. 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 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 wrist pin to be out of alignment, the 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. Only 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, which controls the piston travel path. With the wrist pin bushing bored parallel to the case, the rod can be slightly bent, and it matters none. In effect, the wrist pin bushing may end up thicker on one side and thinner on the other side. It matters none, as the wrist pin 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 reaming the wrist pin bushing. Other issues may exist too.
It is impossible to accurately install and size a wrist pin 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.
An owner that isn’t interested in things being exact may get away with using a ball to size the wrist pin bushing. An engine rebuild is a large job and expensive. Why get cheap with any part of it? It is up to the informed owner to make that decision.
Updated 14 July 2022