Originally Posted by RedRupert
Point taken. I should have used lawn-mower oil (or similar) after the rebuilt, if only to rule out the too slippery oil theory. I'm certainly not saying that I don't agree with you. Here in the Soviet Union, I expect it'll be hard to find proper running-in oil... maybe I should drain it out of a new Yamaha!
Looking at my leak-down results, do you think that ring seating could be the problem?
I think your rings are not seated all the way. With the fairly smooth cylinder surface and the syn oil it will take awhile (if ever), for the rings to take a complete seat. The top rings have the benefit of cylinder pressure to push them into the cylinder wall, the oil rings do not.
I have had a few ring seat issues with some trucks in my fleet. The trucks run propane and since the reformulation of oil here in the US we have had issues with new rebuilds. I had to start using a high zinc oil in combination with a rough hone and higher tension oil rings to solve our problem. I am a believer in the break in oil now.
Regular oil would be a lot easier to try than tearing it down and starting over.