Last night I got my head back together, and tried the plug-the-sparkplug-hole-with-finger test for compression. In the Fred Manual, it says pedal and the compression should force your finger right off. My engine's off the bike, so the only way I could move the piston was to turn the flywheel by hand. It didn't blow my finger off the hole. Either my rings are too old (they look fine, and the bore is blemish free) or you actually need to be turning the piston quickly (pedalling) to get this result. Anyone know which? Turning it slowly by hand I think (I hope) is allowing pressure to "find it's way out" slowly, which pedalling wouldn't allow. I'm going to try to rig up a reverse drill/socket on the flywheel tonight.