It Rolls!

Some pictures of the R51RS project. It is a rolling chassis now. The frame is from a 1952 R51/3 basket case that I came across. It has a new life now!

Comparative Anatomy - Final Drive

I did a comparison of pre and post war plunger frames in a previous post. Here I will compare final drives. I will do other comparative anatomies in the future.

Plunger Frame final drive from 1938 to 1954 are all interchangeable to the extent that anyone of the final drives will fit in any year frame. The notable caveat is full hub final drives only work with the full hub width wheels. But again, a full hub final drive/wheel combination can be used on any year plunger frame.

I have several of these final drives, including a Russian final drive. I thought I would post pictures of each of these and talk about some of the differences. I welcome inputs from others if you have any additional information. I would like this to be a resource for anyone doing their own restoration to know the differences and what would be "correct" for their motorcycle.

Prewar Hub - 

The plunger final drive has the suspension slider cast into the drive cover. There are six bolts that mount the cover to the final drive with the two forward bolts slightly hidden behind the slider. There is a grease port on the slider right above the axle nut.

There is a clear "S" stamped just rear of the axle nut. The story I've been told is that the first castings of the final drive were not up to BMW standards. They change the casting process and the "S" is for the later castings. I've been told that "S" stands for sandcast or in German, sandguss. I have no way to confirm this. If you know have additional information, send me a note.

I also have a Russian version of this final drive.

It is very similar with some notable exceptions. The casting of the is noticeably rougher. There is no "S" but there is a "2" right below the oil fill port. There is a more abrupt edge around the circumference of the drive cover. Also the threaded nut that goes into the drive input (where the drive shaft mates up to the final drive) is different from the BMW. I haven't looked at it very carefully yet, but I believe the thread pitch is different. But in general, it is a good substitute for a BMW final drive if you are missing the original one. I don't know what model this drive came from, but it does not have the two horizontal ribs on the drive cover that I've seen on other Russian plunger motorcycles. I am not very familiar with the differences in the Russian models.

One thing to know is that the number of teeth on the drive spline for a Russian drive/hub is different from the BMW. The BMW has 26 teeth while the Russian drive has 28. So when you see "BMW" half hubs for sale, make sure you ask how many teeth the spline has. It may be a Russian hub and won't work on the BMW drive. Having said that, the Russian spline looks to be more robust than the BMW version. So a Russian final drive/hub on a BMW bike may have some advantages, but clearly it is not original.

Postwar /3 Final Drive - up to 1953

The postwar final drive looks to be identical to the prewar final drive with the prewar drive with the exception of the "S" stamp. Casting is slightly different as you can see some casting lines on the drive cover that is not on the prewar cover.

This is a post war final drive (from a 1952 bike) on my R51RS race bike. The rubber gaiter is not original to the production bike or the R51RS race bike. I just want it to be different.

Full Width final drive - to the best of my knowledge only available in 1954

These final drives look very similar to previous year final drive except they are larger in diameter and have an extra "lip" around the circumference. The front brake plate also has the same "lip" There is also a boss right above the axle nut. Again, these are only compatible with full hub width wheels. These wheels have larger width brake shoes.

I hope you find this type of comparison helpful.