Motor Part 1

I now have my motor, well mostly. The motor is built up minus the ignition and the clutch. I am also getting the tappet tubes sorted out. I have to give a big thanks to Brent for getting the motor to this state. The pistons for this bad boy definitely was a bigger challenge than originally anticipated.


Here is some eye candy:

One of the next steps is to get the gear on the magneto. The shaft on the magneto is too short and I don't think it will hold up under the stresses of a race.

Here are some photos of the engine in the frame:

In a future post, I'll give some details on that transmission case. It is not what it appears.

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.

Progress

It has been about a year since my last posting. Life sometimes gets in the way of, well, life.

Progress has been made. For the frame, I decided not to try to stiffen the prewar frame as was done with the original R51RS. I did not want to modify a rare frame. I was able to locate a 1952 R51/3 basket case. The forks that came with the basket case was a 1954 set of forks. The forks required rubber gaiters. I decided to use rubber gaiters for the front and back, a homage to the pre-war compressor race bikes.

I had some small repairs made to the frame and I had it powder-coated and front forks installed.

The fork tubes were made by Frank's Maintenance and Engineering, Inc. It took almost a year of calls before a production run was made and I got myself a few of the forks for different projects.

 
 

I had some simple aluminum "spools" machined up to allow me to attach the rear rubber gaiters. Those gaiters are Triumph part number 97-1645.

They fit very well and tight! One of the spools had to be heated up a little to get to fit.

I was able to get both front and rear suspension parts all complete

Next step is to get the hubs brake surfaces machined and trued and new brake pads arced. I plan to make the 3 hour trip to Vintage Brakes in the next few weeks and get both the race brakes and the '54 R51/3 brakes refurbished.

Also the motor is nearing completion. I sent out my flywheel to get lighten up and I hope to have a complete motor in the next couple of months. I can't wait.

Comparative Anatomy - Frames

I haven't been able to get a lot of information on the frame used on the R51RS. Clearly it was based on the R51 frame. From photos, the frame an additional brace in front of the rear fender, under the seat. Unlike the brace that was used in R51/3 frames, the brace was centered. The R51/3 brace was off center in order to accommodate the battery. I have some pictures of a couple of frames I own; 1938/1939 frame and a 1954 frame.

First the R51 frame:

 Simple gusset around the steering head

The frame had no bracing in the "interior" volume. Looks to be very flexible, not in a good way.

There is a spot for the battery right in front of the rear fender, on the right side

The R51/3 frame:

 Brace on the across the down tubes, right below the steering head (a little hard to see) and a second one that is easy to see

 Brace from front to the seat post, just as with the post-55 /2 frames

 Brace from top to bottom. Note that the battery box on the '54 frame not only had a "floor" but also a "back". Prior to '54, the battery only had a floor.

Don't mind the bandages. 

I will add that I weighed the two frames, both powder coated. The '54 frame is 5 pounds heavier that the '39 frame. The price of stiffness.

Timing Gears

I want to share some photos from JH who is also recreating a R51RS. From the photos sent to me from JH, he appears to be much further along than I am.

JH took some photos of the timing gears from a generous owner of R51RS who was willing to show what is behind the cover. I really have to thank JH. I spent many months searching the internet for photos of the the R51RS timing gears. The picture I have on an earlier post is the set that Josef Heft sells. I hope to get my set from Josef in the next month or so.

 Copyright JH

 Copyright JH

In the blog post Weiss und Blau, it was noted that in the 1950s, one could have bought the timing gears from Ernst Hoske (known in the /2 world for his tanks). I'm always surprised by the number of R51RS replicas that are out in the world, given that only 17 were only produced and only about five original are know to exist today.