There are big things, like the R12 has two chains to drive the cam and the magneto, while the R52 has 4 gears; and that the R12 has the oddball oil filter in the sump. However, I've found that although the oil pumps are based on the same gear drive design, the R12 has a spring loaded pressure relief valve. (Is it really possible for those little pumps to put out too much pressure?)
One thing in particular that's pretty different is the engine breather. The R52 crank has a hollow nose. At the base of this hollow area is a drilled ring that provides an air path into the hollow area. At the front end of the crank is an oblong hole. The front tip of the crank is installed into a small cap that has a hole at the bottom, and when installed, this hole lines up with the breather pathway. So, as the crank rotates, the oblong hole spins into alignment with the hole in the cap and allows the engine to breathe out; then it spins out of alignment and the crankcase is closed again. Although it takes a lot of words, it's really very simple.
The R12 has a very different arrangement. A separate piece spins with the sprocket on the front of the cam shaft. This piece has a hollow nose with two holes, 180* apart. There's a cover over this piece that closes the crankcase, but on the inside it has a complex casting that includes the same kind of cover with a hole. The hole lines up with a pathway in the top crankcase half casting, which in turn opens to a cap that is in front of the crank. The cap is closed to the inside of the crankcase, however, and instead is open to the front. A banjo bolt threads in and hold a short pipe that directs the breather outlet to the side of the motor.
Why did BMW go to all this effort to change what was a beautifully simple and straightforward breather? What's the advantage of all this extra machining and the necessity for careful alignment?
The only thing that has occured to my dull wit is that the breather on the R12 spins at half the speed of the one on the R52, and so perhaps there is less wear as the very close fitting spinning hole goes around in the cap with the fixed hole.
Can someone set me straight about this?