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;
- Rockers pivot on a central shaft and the clearance is adjusted by means of screw and locknut.
- Each rocker is pivoted on its own pillar and the clearance is set by turning the adjusting nut to alter the position of the rocker on the pillar.
- Some overhead-cam layouts use fingers followers, in which case the clearance can be adjusted by means of a nut at the finger’s pivot.
- Some direct-acting overhead-cam systems use an adjustable shim, whose thickness can be altered by means of conical screw.
- Others such as overhead-cam systems use interchangeable shims, so adjusting the valve clearance calls for extensive dismantling.
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.