Another method is to push several feet of small-diameter rope into the spark plug hole and turn the crankshaft to cram the combustion chamber full. No way for a valve to fall into the cylinder that way, and no need for air pressure! About 10 feet of 1/4" rope is required.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!