You're not going to want to hear this, but I'm convinced that I destroyed a Metra 65 cylinder by not breaking it in properly.
It had an airleak in the beginning, which didn't help. I managed to sieze up the engine, then I fixed the air leak and got the jetting about right.
I was running a Homoet 6P exhaust. The powerband was up pretty high, and I ripped on that new cylinder pretty hard. I wound the engine up pretty high to see how fast it would go for about a mile, and it never really ran properly after that. Everything would get really really hot, and I could hear noise coming from the top end. It was a tapping at idle, and in the powerband, it sounded like the piston was hitting the head.
I switched to a bigger jet, and ran so rich that the spark plug was wet, and it still overheated.
I retarded the timing from 2mm before TDC, to 1mm before TDC, to no advance. It lost some power, but still ran hot.
I reduced the compression, and that didn't help either.
It would overheat no matter what I tried. Too much friction? It got really hot. The spark-plug boot was melting.
...and you can still see the cross-hatching on the cylinder walls.
I'm taking it easy on the new kit. Compared to the old one, it's really quiet, and runs cool as a cucumber : /