The difference between a plain bearing engine and a ball bearing engine:

About 300RPM.

Plain bearing:
bronze ferrite BUSHING to support the crankshaft in the crank case

Ball bearing
steel balls inside a race--pressed into the case--supports the crankshaft

ABC/ABN/ABL:
piston without a ring--chrome sleeve--tapered at the top

Ringed:
piston with a ring--chrome sleeve--straight bore