Valve Rotation

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.

As the valve opens and closes, it has a natural tendency to rotate, very gradually, so that it keeps seating in a new place. This produces a slight wiping action which helps keep the face and seat free of carbon. It also helps prevent sticking in the valve guide and distributes heat around the valve seat.

On some diesels, the inlet valve has a shroud or mask on the back of the valve head. This is designed to cause turbulence in the incoming air. The position of this mask is critical for best operation, so the valve is pinned to prevent it rotating. If the rocker arm is slightly offset to the valve stem, it can help this rotation.

Some engines even use positive valve rotators. The valve operates in a valve guide and it is exactly concentric with the valve seat. The valve guide is the hollow cylindrical part in which the valve stem moves.

The valve guide area can be machined from the metal of the cylinder head, or holes can be drilled for pressed-in guides. Cast-iron guides are necessary in aluminium-alloy heads to provide a suitable bearing surface for the valve stem.

Many heads use replaceable valve guides that are a form of metal bush pressed into holes in the cylinder head.

Other cylinder heads have guides cast as part of the cylinder head, then bored to the size of the valve stem during manufacture.