seat contact is very narrow and located close to the
bore. The grinding-in, using finer and finer
compounds as the work progresses, causes the seat
contact to become broader. The contact area should be
a perfect ring covering approximately one third of the
Be careful that you do not overgrind a seat or disk.
Overgrinding tends to produce a groove in the seating
surface of the disk. It may also round off the straight,
angular surface of the disk. Overgrinding must be
corrected by machining.
When a valve seat contains irregularities that are
too large to be removed by grinding-in, you can
remove them by lapping. A cast-iron tool (LAP) of
exactly the same size and shape as the disk is used to
rule the seat surface. Two lapping tools are shown in
Here are the most important points to remember
while using the lapping tool.
Do not bear heavily on the handle of the lap.
Do not bear sideways on the handle of the lap.
Change the relationship between the lap and the
seat, so the lap gradually and slowly rotates
around the entire seat circle.
Keep a check on the working surface of the lap.
If a groove develops, have the lap refaced.
Always use clean compound for lapping.
Replace the compound often.
Spread the compound evenly and lightly.
Do not lap more than is necessary to produce a
smooth, even seat.
Figure 4-12.Lapping tools.
Always use a fine grinding compound to finish
the lapping job.
When you complete the lapping job, spot-in and
grind-in the disk to the seat.
Use only approved abrasive compounds to
recondition seats and disks. Compounds for lapping
and grinding disks and seats are supplied in various
grades. Use a coarse grade compound when there is
extensive corrosion or deep cuts and scratches on the
disks and seats. Use a compound of medium grade to
follow up the coarse grade. It may also be used to start
the reconditioning process on valves that are not
severely damaged. Use a fine grade compound when
the reconditioning process nears completion. Use a
microscopic fine grade for finish lapping and for all
Badly scored valve seats must be refaced in a lathe
with a power grinder or with a valve reseating
machine. Use the lathe, rather than the reseating
machine, to reface disks and hard-surfaced seats.
Work that must be done on a lathe or with a power
grinder should be turned over to machine shop
personnel. This discussion applies only to refacing
seats with a reseating machine.
To reface a seat with a reseating machine (fig.
4-13) attach the correct 45-degree facing cutter to a
Figure 4-13.Valve reseating machine.