RE: Cox Reed Valve FAQ
I will present this as a observation, not as a recommended method:
If one has an engine with the standard TD type aluminum spinner, one can often just pinch the spinner between ones thumb and the side of one's index finger. This seems to only work well if the engine is running a bit rich and/or cold, and has not gotten too much RPM...
Generally, the big concern with any brute force method of shutting down the engine by stopping its rotation is not damage to the engine, but damage to the airframe. This is a concern when using a rag in the prop. Perhaps using the hollow end of a chicken stick on the spinner to stop the engine might be a bit safer for the operator than just using fingers. This method puts less stress on the equipment than a rapid stop from a rag. If we are talking about something fairly overbuilt like a trainer or most any kit built C/L model though, the rag in the prop likely won't harm anything.
Arlen