I have a OS .40FSR with a ring. Does a ringed engine have less
trouble going over the top when cold than a ABC? Mine has a
little resistance when I reach compression as opposed to a ABC
engine which has lots of resistance at compression when cold.
The FSR has been sitting for 4 months without running. How much
does having fuel up in the cylinder have to do with how much resistance I'll feel? If the cylinder is pretty dry, then does the ring
not create a tight-enough seal to create this resistance? Just wondering, because I remember this engine having more compression "feel" than it does now.

Jesse

Jesse, everything you said is correct. Ringed engines don't have a very good seal until their running, where combustion helps seal the ring and there is less time for it to leak past the ring. Ringed engines do seem to last a lot longer than an ABC or ABN and require a lot more break in time too. They will survive a lean run better too than an ABC engine.
Suptter

I just received a ringed four stroke - used, and I was very disappointed in the compression. As soon as I ran it I was OK with it because it's running very well and strong. I didn't realize there would be that much difference between abc and ringed. I don't believe my old Supertigre 2 stroke is as weak as that cold, but this 4 stroke is.

4*, did you check the valve lash? My four stroke will run and even fly the plane with the valves slightly out of adjustment but there's a noticable loss of compression when flippng the prop and it's just a little harder to start by hand.

FSR's used two types of ring. If it's a Dykes ring then don't expect much compression turning it over by hand because it needs gas pressure getting behind the ring to push it hard against the liner. If it's a normal ring then it'll give fair resistance to compression. Neither will feel like an ABC because an ABC piston is actually bigger than the liner near the top (an interference fit).

You can tell what type of ring it has just by looking through the exhaust port. The Dykes ring will seem to sit on top of the piston so it's flush with the top surface but the normal ring will be about the width of the ring down from the top of the piston. Don't be put off because it's not ABC, I'd have a ringed engine any day