DOHC 16v refers to a 4-cylinder internal combustion engine with 4 valves in each cylinder and two camshafts. The reason for the use of two camshafts is that the engine uses four valves in a cylinder - 2 valves for the intake and the other 2 valves for the exhaust. As a result of this number of valves, the air distribution is faster so that the engine RPM can easily reach its maximum.
A camshaft is driven by the crankshaft of the engine and it opens the intake and exhaust valves so that the engine can take air in and expel the exhaust gases. It is installed on the cylinder head so that it will directly press the valve.
In some types of DOHC 16v engines, Rocker Arms are used in adjusting the valve gap. DOHC mechanisms that use Rocker Arms have a hydraulic lash adjuster system to automatically adjust the valve gap. For those without Rocker Arms, the valve gap is adjusted manually using a ring that is placed inside the valve lifter. To change the valve gap, this ring must be replaced.