Valve Guides

The valve stem moves in a valve guide, which can be an integral part of the cylinder head " or
engine block for L- head engines ", or may be aseparate unit pressed into the head ( or block ) .

Guides are a push or press fit in the cylinder
head,for ease of replacement when worn.They
are usually of cast iron, favoured because of its
good bearing properties.

In some early engines the valves were carried
simply in holes in the cylinder head ,but
rectification was difficult when the bores of the
holes wore.

The valve rotator turns the valves a few degrees
on opening to wipe the Valve seat , avoid local
hot spots , and prevent deposit building up in
valve guide.

Valve rotation can wipe deposits off the seat ,spread lubricant, solid or liquid , around the seating
faces, and distribute wear and pitting uniformly around the seat, thus ensuring that the heat flow is
not impaired locally by either wear or distortion.

Excessive rotation should be avoided, however, because it can actually cause wear, especially if
there is a tendency to local welding.

A common method of rotating valves is to place a hard steel thimble over the end of the stem,seating
its rim on the retainer washer for The spring, so that there is a small clearance between the tip
of the stem and the other end of the thimble.When the cam or rocker bears down on top of the thimble
it first compresses the spring, momentarily freeing the valve while the clearance between the tip
of the stem .