I guesstimated a lifter is 3/4” diameter. Radius would be 3/8”. .375XPi R square is .44Sq inches. 100PSI X .44 is 44 pounds force. I later measured a GM SB lifter. The actual radius of the lifter piston is .657”. That works out to 33 pounds of force at 100PSI.... Login to read more.