Valve Clearances
Downwind , assuming that this is a genuine question and not just a windup,
a. hydraulic lifters (tappets), unlike solid lifters, do not require adjusting once set up and generally will run much quieter than solid lifters. They are however suceptible to problems caused by dirty oil.
b. Insufficient clearance on an inlet valve will allow leakage during compression stroke and combustion stroke.
c. Poppet valves actuated by solid lifters and pushrods require clearance to allow for metal expansion and so prevent valve burning and/or gas leakage.