Overhead valves allow fuel to enter while allowing exhaust to exit. They are installed within the cylinder head. They are opened by either a camshaft in the cylinder box or one or more camshafts in the cylinder head, which may act directly on the valve or indirectly via rocker arms. When the valve has expanded due to heat, it is critical that the valve springs can close it properly in all circumstances.
If the clearance is too small, the valve may slightly open and burn; if it is too large, gas flow will be obstructed, and the valve gear may be very noisy. Removing the cover from a single-overhead-cam engine with finger followers interposed between the camshaft and valves, adjustment with the finger followers is similar to that with rockers: an adjuster is turned at one end (in this case with a spanner), while the clearance is measured at the other.
There are five different provisions for valve clearance adjustment;
The most common method of adjusting valve clearance, with an adjusting screw and locknut at one end of a rocker. Turning the screw until the clearance, measured with a feeler gauge, is recommended with the ‘opposite’ valve fully open.