I think there is some mis-understanding. The engine did not run whell when I mixed some left-over 10% fuel with some 5% fuel at about 50/50. I think this would give ne a net nitro content of about 7.5% or thereabouts. The engine did not run well at all on this.

When I went back to 5% nitro fuel it was a happy engine.

From this I learned that 5% is the MAX nitro content to run on this engine.

I did take it apart while fitting it to the airplane. There is one aluminum gasket/shim in place from the factory.