A1: Engine Repair

They support and align the valve stem, keeping the valve centered on its seat and controlling oil down the stem.

With a dial indicator on the valve head while rocking the stem, or by measuring stem diameter and guide bore with a small-hole gauge.

It provides the hardened mating surface the valve face seals against to seal the combustion chamber and transfer valve heat to the head.

It is the band where valve and seat touch; too wide reduces sealing pressure, too narrow causes burning and poor heat transfer.

It closes the valve and keeps the lifter, pushrod, or follower in contact with the camshaft, preventing valve float.