RE: FSR Ringed?
MJ,
Most original OS FSR (and RSR) series engines had a ringed piston (smaller ones were lapped cast-iron+steel).
Only in 1978-79 did OS begin to offer "ABC" (actually, ABN) versions of some FSR/RSR/VR engines.
The ringed version has a significant advantage that it cannot peel the sleeve, but the "ABC" made a bit more power...
Ringed and ABN version of the same engines were sold side-by-side.