4 – VALVE TRAIN
49
Piston Engine Fundamentals
TC010-05-01S
VALVE GUIDES
Some valve guides are machined directly into the cylinder head casting.
Other guides are soft alloy inserts that are pressed into the head, as
shown in Figure 33. In some cases, the valve guide is reamed (drilled out)
for a close fit to the valve stem after the guide has been installed.
The valve guide fits very closely around the valve stem, with just enough
room for lubricant and free movement of the stem. When the guide wears
out, it can sometimes be reamed out so that a valve with an oversized
stem can be installed. Insert-type guides are replaceable.
FIGURE 33. Valve
guides hold the
valve stem so the
valve fits
squarely on its
seat.
Cylinder
head
Valve stem
Valve guide
Valve seal