Hi Ed: I have two .46's and one .39 JBA engines. The .39 is the only one having gone through the break-in. It is mounted in an old Royal trainer right now waiting for test flying. While on the test stand, this .39 engine with a 9x5 prop and 10% castor oil fuel ran over 18,000 at the rich 2 cycle break point. The muffler does not keep the noise down very well though. My inspection of the engine before running found no bad discrepancies although I thought the c/s drilling for the carb induction a bit wrough. The .56 engine may perform as well as your SK .50.

Rich S.