A narrow valve seat is desirable because a thin circular contact with the valve face forms an efficient seal. But a wider seat transfers heat better from the valve to the cylinder head.
In some cast-iron cylinder heads, valve seats are cut directly into the edge of the valve port. Valve seat inserts are metal rings that match the shape of the valve.
An operating valve tends to rotate and keep seating in a new place. This helps keep its face and seat free of carbon, and reduce sticking in the valve guide. It also spreads heat around the valve seat.
Coil springs normally hold valves on their seats. Their coils are usually closer at the bottom than the top. Oil seals on valve stems or guides prevent too much oil passing into the combustion chamber.
Intake valves are usually larger than exhaust valves because pressure forcing charge into the cylinder is much lower than that forcing the exhaust gases out. Different engines use different valve combinations.
An overhead valve or pushrod system has the valves in the cylinder head and the camshaft in the block near the crankshaft. Many engines use hydraulic valve lifters to quieten the engine and eliminate valve clearance adjustments.
The time valves in a 4-stroke engine cycle actually open and close can be measured by angles. These angles can be easily read using a valve-timing diagram.
Valve overlap is the amount of time the intake and exhaust valves are both open at once. Less overlap produces a smooth idle and more slow speed torque. More valve overlap allows better engine breathing at high speeds.
