In a four stroke engine for each crank shaft revolution, the camshaft revolves one half turn.

A camshaft on a four stroke engine runs at 1/2 the crankshaft speed , the intake and exhaust valves opens once (each) for each two engine revolutions. In a two stroke engine, it is one to one.

The timing belt is attached to the camshaft pulley and timing belt drive pulley.

The motion of the cam is transferred to the valves through rocker arms.

Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event.

Valve overlap is the number of degrees of camshaft rotation during which both valves are open.