This picture of a cylinder shows the exhaust valve stem
guiding bore. When adapting the cylinder to take an exchangable screwed-in
guide, the protuding part of the old guide should be cut-off before drilling
of the valve stem bore is carried out.
This Figure shows the adaptations for centred fitting
(see text) of the screwed-in guide, based on the guide TRW 81-2696. The
screw thread selected is spark plug thread (14x1.25 mm) for providing
optimal wall thickness proportions of the guide and the remaining cylinder
material around the guide bore.
(The small screw for locking the valve guide is not shown).