Originally Posted by
wingracer
There were two Mugen M 12's I believe. The original MT-12 and the later MR-12. I believe both had the M on the crankcase. The main difference was the piston and the length of the rod. The MR-12 will have a cutout on the bottom of the piston skirt for clearance with the crank. The MT had a longer rod and so it did not require the cutout on the piston.
Yep, you are right. it's been a while with these two engines
The quickest way to keep them apart without opening them is to look between the crankcase and the cooling head.
On the MT 12 the sleeve sits completely on top of the case and on the MR12 it sits in a reset like on all the later engines.