Silvaire,

I'd heard somewhere that the .15 rear rotor was almost an accidental. ST had started to develop a .10 glow, front intake, sport engine, but changed plans. The back end of yours, allegedly, was to have been the front end of the other engine. More of a market for a high performance .15?

BTW, mine has a brass plug in the end of the 'crankshaft' at the back... Amazingly noisy engine, and turns fast. Wouldn't, and didn't, use it much - very scarce, and ST even then had a reputation for scarce parts for even popular engines...