This is a question I hope Dave will be willing to answer.

I've had 6-7 YS engines in the last 10 years. That includes the original 120SF, 120AC, 140FZ and the 140L. They have all been put to hard use in pattern planes.

I've been very pleased with the overall performance and handling of these engines. However, they've all had a gradually performance degradation that I think is starting after about 80-100 flights.

After break in, the compression is usually excellent. After about 80-100 flights the compression is reduced and it is possible to hear the engine leaking when turned over slowly. In my opinion this is due to leaking valves and not the piston ring, as grinding the valves seems to be the only cure.

The valves in Saito and OS engines seems to last much longer than this. I've heard that the newest YS heads has aluminum valve seats as opposed to the typical brass inserts.

Can anyone (Dave ?) comment on this performance degradation and the longevity of the valves and seats?

Oistein, NORWAY