So the part numbers are as follows...

11217546618 - R53
11217508547 - R50

So on the catalog there's a difference; but is there really a difference?

CR Bearing shells are still 44.0mm
Main Bearing shells are still 48.0mm

They share the same Impulse Wheel (11217513056).
They share the same Lower Timing Sprocket (11211485402)

I came up on an R50 engine with bad compression for dirt-cheap... so I snatched it up with intent of building a block on-the-side to eventually put it in my R53.

 

My understanding is that the R53 crank is forged, while the R50 crank is cast. Pistons and rods are different, too, as are the valves, and the R53 has oil squirters in the block that the R50 doesn’t.

 

The oil squirters are easy enough to tap into the block... and they're widely used BMW part. (11117524800)

The head is going to have different valves, so that's irrelevant. The rods and the pistons are also going to be replaced.

THE CRANK... now that's the hard part. I don't really feel like spending $500 for a used crank that may or may not legitimately be from an R53.

 

Yes the R50 and R53 cranks are different.
R50 is cast
R53 is Forged

Also they are different sizes as the R50 is actually fatter on the snout as you can't put an R53 crank pulley on and R50.

So if you are trying to build and S engine you need and S crank