mike -- Hydraulic lifters run at zero clearance all the time. They don't force the valve open unless they're on the raised area of the cam lobe as the shaft turns. At 2500 RPM each valve is lifting and closing 21 times a second- - - -no time for the slight leakage built into the hydraulic lifter to have any effect on the operation of the engine!
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!