The RS engine, part 2

The R51RS in 1939 followed the R51SS of 1938 and R5SS previous to that. The R51SS was also based on the production R51 with more modest tweaks than the RS. The SS used the standard chain driven cams that the production bike used. It also used the production ignition system. Some of the 1938 SS modifications included high crown pistons which increased the compression ratio from the stock 5.6:1 up to 8:1. Larger carburetors, 25.4 instead of the 22 mm. The alternator robbed less power from the engine by using a 14 tooth gear instead of the stock 12 tooth gear. About 50 of the SS versions, both R5 and R51, were built.

Beppe Stelzer, lead of the BMW race department, oversaw the R51RS changes with the replacement of the  chain drive with a intermediate helical spur gear. The battery was eliminated with the used of a racing magneto, Bosch's W2R48. The used of the magneto required the top of the engine case, where the alternator typically is housed, to be machined flat.

The production round iron cylinder was replaced by an aluminum "porcupine" casting, similar to that used by the BMW R66. Of course, an original porcupine cylinder is impossible to find. Fortunately one can get reproduction cylinders from SMA Racing in Germany. This is where I got my cylinders. Note that SMA Racing has some very cool RS54 engine and transmission casings as well. Maybe for my next project. I'll start saving my pennies.



SMA Racing also produces R51 heads and valve covers that can be used for a R51RS project. I fortunately was able to find some original heads and valve covers. The R51RS used magnesium valve covers which can be gotten from SMA Racing. I may go down that path as well, but I have a ways to go before I have to  make that decision.

Note that all these parts (and a lot more!) can be gotten through Josef Heft. A quick google search will show the massive extent of parts he has to offer. For me, I only embarked on this project because Josef had a very comprehensive catalog of parts.

Let's finish up on the engine. The R51RS used the standard 300 degree cam opening but with a 6.5mm lift. This translated to a valve lift of 10 mm instead of the stock 5.8 mm. As with all race engines, polishing and balancing was employed. A lightened flywheel was used and an air-scoop was used to keep the clutch cool. The air-scoop was located where the timing hole would be used in later models, right behind the left carburetor. You can see the polished air-scoop in the pictures from the BMW museum.

The RS produced 12 more than the production engine. At 36 hp, that is a 50% increase over stock!

I should note that most of the technical information I have on the R51RS came from a 2002 article in Motorrad Classic as well as internet searches.