Double or dual overhead cams, (DOHC), refer to individual camshafts used to operate the intake valves and exhaust valves. This allows greater flexibility in placing the valves in the combustion chamber at the optimum angle for maximum power. Separate camshafts allow the use of a lightweight and compact valve actuating system, which is suitable for high RPM operation.
On most Yamaha DOHC models, a chain driven by the crankshaft rotates the camshafts. DOHC systems where the camshafts operate directly on the valves use shims to maintain the valve adjustment while other DOHC systems use the more conventional rocker arm mechanisms between the camshaft and valve.