[jap thrasher]

Having read so many different reasons why valve wear increases or decreases valve clearance. I was surprised that no one explained the reason why.
There are three different ways to operate the valve train in an engine.
The most common type for years was the pushrod type found in most forms of high performance car engines. When flathead -- flathead came from the look the engine had, (L head combustion chamber.)-- type engines in cars went bye bye after WWII, because the valves were located in the block wtih the pistons and cylinders, the valve sank into the block and not the head therefore decreasing valve clearance. In the pushrod engine (known as the overhead valve type), because the valve train and rocker arms that pivot when the pushrod pushes up from camshaft to open valves, the camshaft was still below the valves. Since the valves were in the head and not in the block like the flathead. The overhead valve train wears differently because of the pivot wear. so valve clearances increase.
Most of our real perfomence bikes are called overhead camshaft (meaning the cam runs over the top of the valves and in most cases in direct contact with the camshaft thus eliminating the overhead valve train parts) because the valve will always wear into the seat and head direction thereby decreasing clearance.
Adjustment of valves becomes critical as the ratio of horsepower to cc increases and the rpm's go up as they do today.
--jap thrasher