proper clearance in valve inside the engine
Too much clearance means the valves will likely clatter and, over the long term, cause damage to the valves, camshaft lobes or rocker arms. If there’s too little valve clearance, the valves won’t fully close, causing excessive heat, and the engine will lose power.
Opinions may vary between piston and valve manufacturers, but a popular consensus is a minimum clearance of .080-inch for the intake and .100-inch for the exhaust. The exhaust valve expands more due to heat from combustion, and therefore needs additional clearance.