To grind-in a valve, first apply a light coating
Spotting-In Valves
of grinding compound to the face of the disk.
Then insert the disk into the valve and rotate the
The method used to visually determine
disk back and forth about one-quarter turn; shift
whether the seat and the disk of a valve make
the disk-seat relationship from time to time so the
good contact with each other is called spotting-
disk will be moved gradually, in increments
in. To spot-in a valve seat, you first apply a thin
through several rotations. During the grinding
coating of Prussian blue (commonly called Blue
process, the grinding compound will gradually be
Dykem) evenly over the entire machined face
displaced from between the seat and disk surfaces;
surface of the disk. Insert the disk into the valve
therefore, you must stop every minute or so to
and rotate it one-quarter turn, using a light
replenish the compound. When you do this, wipe
downward pressure. The Prussian blue will adhere
both the seat and the disk clean before applying
to the valve seat at those points where the disk
the new compound to the disk face.
makes contact. Figure 6-16 shows the appearance
When you are satisfied that the irregularities
of a correct seat when it is spotted-in; it also shows
have been removed, spot-in the disk to the seat
the appearance of various kinds of imperfect
in the manner previously described.
seats.
Grinding-in is also used to follow up all
After you have noted the condition of the seat
machining work on valve seats or disks. When the
surface, wipe all the Prussian blue off the disk
valve seat and disk are first spotted-in after they
face surface. Apply a thin, even coat of Prussian
have been machined, the seat contact will be very
blue to the contact face of the seat and place the
narrow and will be located close to the bore.
disk on the valve seat again and rotate the disk
Grinding-in, using finer and finer compounds as
one-quarter turn. Examine the resulting blue ring
the work progresses, causes the seat contact to
on the valve disk. The ring should be unbroken
become broader. The contact area should be a
and of uniform width. If the blue ring is broken
perfect ring covering about one-third of the
in any way, the disk is not making proper
seating surface.
contact with the seat.
Be careful to avoid overgrinding a valve seat
or disk. Overgrinding will produce a groove in the
Grinding-In Valves
seating surface of the disk; it will also round
off the straight, angular surface of the disk.
The manual process used to remove small
Machining is the only process by which over-
irregularities by grinding together the contact
grinding can be corrected.
surfaces of the seat and disk is called grinding-
in. Grinding-in should not be confused with
refacing processes in which lathes, valve reseating
Lapping Valves
machines, or power grinders are used to re-
condition the seating surfaces.
When a valve seat contains irregularities that
are slightly larger than can be satisfactorily
removed by grinding-in, the irregularities can be
removed by lapping. A cast-iron tool (lap) of
exactly the same size and shape as the valve disk
is used to true the valve seat surface. The follow-
ing are some precautions you should follow when
lapping valves:
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 valve seat occasionally so that the
lap will gradually and slowly rotate around
the entire seat circle.
Figure 6-16.--Examples of spotted-in valve seats.
6-12
