ORIGINAL: Bill Adair
I'm certainly no engine expert, but I mentioned this before.
I have a Norvel Big Mig Revlite .061 that has exactly the same loss of compression, and I think it's because I attempted to do the run-in with the NV diesel head installed. It should have been run-in on glow fuel first, with a smaller prop to allow it to rev up quickly, as the instructions suggest.
Norvels are known for their tight piston fits, and some of mine have special instructions on how to free them up before the first runs, by turning them over many times with the head removed, and lots of lube on the piston. I always considered that as foolishness, because it's almost impossible to get them up to running temp by flipping the prop, or even using a drill press to spin the engine over.
My theory is that performing the run-in as a diesel, does not allow them to reach normal operating temperatures, and particularly with the Revlite cylinders! The larger fins give lots of extra cooling, but running as a diesel at even lower than normal temperatures, never allows the piston/cylinder fit required for proper run-in. The results may be very rapid wear of the piston, and/or cylinder walls, not to mention possible big end wear!
Martin, if you bought that replacement cylinder/piston set from NV, it's possible they will send you a new set. Sure worth asking.