The prop strike occurred while the engine was fitted with a toothed belt reduction unit. This I later removed because it was so poorly manufactured the belt would run off its cogs and there was no way to align it. I don't think that the shock transferred to the crank indirectly via toothed belt should account for failure of the crank. You stated that some crankshafts were manufactured from crook steel - ergo the failure in my engine. I am not sufficiently in my dotage not to recall the frightening events of this time. It was probably on Ron Lang's advice that I fitted the exhaust seeing that he did not have an exhaust system supplied with the engine. I don't wish to open old wounds or criticise the guy for what happened so long ago. I accept that, in those days, we were all experimenting and that this was the way it was. I never flew at Pakenham and was fortunate, as were a lot of others, to come through those days alive. However I don't want anyone to remain under the illusion that this was a fit device to power any ultralight. I recall that Ron Firth, from around Bowral, had a Skylark with cast iron barrels and direct drive on his Scout and I never heard that he got anything but good service from it. Regards, Don