Engine Valves

A valve is a device for controlling flow through an opening. The internal combustion engine, which is basically an air pump, depends on the efficient sealing of the valves in order to produce compression. The timing of when the valves open, and the duration of their opening, affects engine operation. The cam dictates these two factors. The following terms describe the major components associated with the valves. "Valve clearance" is the gap between the end of the valve stem and valve lifter or rocker arm to compensate for expansion due to heat. Engines with hydraulic lifters often do not need valve clearance adjustments because the lifters automatically take up the slack. The "valve face" is the part of the valve which mates with and rests upon some seating surface. The "valve head" is the portion of a valve upon which the valve face is machined. The "valve lock" (also called the key, keeper, or washer) is a device which holds the valve spring in place on the valve stem. "Valve overlap" is an interval which is expressed in degrees where both valves (intake and exhaust) in each cylinder are open at the same time. The "valve seat" is the part of the cylinder head upon which the valve face rests. These are precision ground to mate with the valve face and thereby seal the cylinder. The "valve spring" is attached to the valve to return it to its seat after lift is released. The "valve stem" is the longest portion of the valve which passes through the valve guide. The "valve guide" is the sleeve through which the valve stem passes. It is pressed or threaded into the cylinder head, and is self lubricated by the composition of its materials. Older cars depend on the lead in "Regular" gas to lubricate the guides. The "valve timing" refers to the relative position of a valve (either open or closed) to the piston in its travel, in crankshaft degrees. The "valve train" is the complete set of mechanisms used to transmit the rotating motion of the engine crankshaft to the reciprocating valve stem, causing the valves to open.