Grinding, by removing metal from the face, makes
the valve stem extend through the head more. This
affects spring tension and rocker-arm geometry. Grind
the face of the valve as little as possible. A sharp valve
margin (fig. 3-58) indicates excessive valve face
removal and requires valve replacement. If the margin
is too thin, the valve can burn when returned to service.
It may not be thick enough to dissipate heat fast enough.
The head of the valve can actually begin to melt, burn,
and blow out the exhaust port. Refer to the
manufacturers manual for specifications about
minimum valve margin of thickness.
If the head of the valve wobbles as it turns on the
valve grinding machine, the valve is either bent or
chucked improperly. Turn off the machine and check for
causes. If the valve is bent, replace it with a new one.
If a burned valve is not noticed during initial
inspection, it will show up when excess grinding is
required to clean up the valve face. A normal amount of
grinding does not remove a deep pit or groove. Replace
the valve if it is burned.
There are several satisfactory methods of checking
for valve guide wear. One procedure for checking valve
guide wear is to slide the valve into its guide. Full it
open approximately 1/2 inch, then try and wiggle the
valve sideways. If the valve moves sideways in any
direction, the guide or stem is worn Another checking
procedure involves the use of a small hole gauge to
measure the inside of the guide and a micrometer to
measure the valve stem; the difference in the readings is
the clearance. Check the manufacturer's manual for the
maximum allowable clearance. When the maximum
clearance is exceeded the valve guide needs further
servicing before you proceed with the rest of the job.
Servicing procedures depend on whether the guide
is of the integral or replaceable type. If it is the integral
type, it must be reamed to a larger size and a valve with
an oversize stem installed. But if it is replaceable, it
should be removed and a new guide installed
Another area on the valve that must be attended to is
the valve stem. This is due to wear from the valve
operating mechanisms. When the tip end of the valve
stems is rough, smooth them by grinding lightly with a
special attachment furnished with the valve grinding
machine. Grind as little off the stem as possible. Many
stems are hardened and too much grinding results in
rapid wear when the valve is returned to service.
Generally, cut the same amount of metal off the face and
stem. This helps to keep the valve train geometry
KNURLING of the valve guides has become more
popular as a method of compensating for wear of the
valve guides. Knurling is accomplished by attaching a
special tool to an electric drill and inserting the tool in
the worn guide. This method is not recommended if the
guide has been worn excessively or knurled previously.
Valve guides should be removed and replaced with
special drivers (fig. 3-59). When working on a valve in
the cylinder head of an engine, you may use an arbor
press to remove and replace the valve guides.
Valve Guide Service
After the valve guides are serviced and the valve
seats are ground, check the concentricity of the two with
a valve seat dial indicator (fig. 3-60). Any irregularity
in the seat will register on this dial.
Servicing of valve guides is an important, but often
neglected, part of a good valve job. The guide must be
clean and in good condition before a good valve seat can
be made. Valve guide wear is a common problem; it
allows the valve to move sideways in its guide during
operation. This can cause oil consumption (oil leaks
past the valve seal and through the guide), burned
valves (poor seat to valve face seal), or valve breakage.
Valve Seat Service
Figure 3-58.Proper valve margin of thickness after refacing.
Valve seat service requires either replacement of the
seat or reconditioning of the seat by grinding or cutting.
Valve seat replacement is required when a valve seat is
cracked, burned, or recessed (sunk) in the cylinder head
Normally, valve seats can be machined and returned to
To remove a replaceable pressed-in seat, split the
old seat with a sharp chisel. Then pry out the old seat.
New seat inserts should be chilled in dry ice for about 15
minutes to shrink them, so they can be driven into place
easily. The seat expands when returned to room
temperature, which locks the seat in place.
In most cases, the valve seats are not replaceable, so
they must be ground (fig. 3-61). Before operating the
valve seat grinding equipment in your shop, be sure
to study the manufacturers manual for specific