Rolf,
The two cycle engines of the mid seventies was governed by several factors. First in the late
sixties the piston port gave way to the reed valve, the rotary valve, and the "case reed" valve.
The leakdown of engines depended on where you measured the leak. If you measured the leakdown with
the ring above the ports with a carefully fitted ring (hand fitted) about 8-11% with a iron/steel
liner. The later all aluminum coated bore cylinders could get in the <5% range like a 4 cycle.
Most ran a single pinned ring. Seals of the crankcase are another story. This is off topic so I'll
cut it short. The rotary seal problems of the early engines reminded me of the 2 cycles though.
Bill Jepson

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