Rope safe and foolproof...

1. Air potential for accidental turning of the crank ( computed this way : your piston diameter times 3.1415 times the number of pounds of pressure you apply to the top of the piston ( then at half a rotation multiplied by the length of the crank throw )... if perfectly centered ... fine... if not it turns the crank violently....
2. Ability of air to hold the valves up depends on PERFECT seal at the valve lip... one SPECK of carbon or burned area on valve and you drop the valve into the bore....

Diesel guys don't have this problem... piston gets so close to head with the 21 to one compression ratio the valve can rest on the piston...