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Valve Guide and Valve Rotation
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 keeps the face in the seat free of 00:27. It also helps prevent sticking in the valve guide and this 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 for right tighting. If the rock around is slightly offset to the valve stem, it can help this rotation. Some engines even use positive valve right tighters.
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 pre-steam guides.
Cast iron guides are necessary in aluminum alloy heads, to provide a suitable barring surface for the valve stem. Many heads used replaceable valve guides that will be a form of metal bush pressed into hollows into the cylinder head. Other cylinder heads have guides cast as part of the cylinder head then board to the sides of the valve stem during manufacture.
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