Figure 3-62.Self-centering pilot.
instructions. The following procedures are typical for
grinding valve seats:
Select and install the correct size pilot (metal
shaft that fits into the guide and supports cutting
stone or carbide cutter) (fig. 3-62). The pilot
should fit snugly in the valve guide and not
Select the correct stone for the valve seat. It must
be slightly larger in diameter than the seat and
must have the correct face angle. Slip the stone-
and-sleeve assembly over the pilot.
Insert the power head into the sleeve assembly.
Support the weight of the power head. Grind
only long enough to clean up pits in the seat.
Check the progress often to ensure that you do
not remove more material than necessary to get a
After grinding valve seats, it is recommended that
you lap the contact surfaces of the valve and valve seat.
Lapping valves are done to check the location of the
valve-to-seat contact point and to smooth the mating
To lap the valve, dab grinding compound (abrasive
paste) on the valve face.
Install the valve into the
cylinder head and rotate with a lapping stick (a wooden
stick with a rubber plunger for holding the valve head).
Rub your hands back and forth on the lapping stick to
spin the valve on its seat. This rubs the grinding
compound between the valve face and the seat. Remove
the valve and check the contact point. A dull gray stripe
around the seat and face of the valve indicates the valve-
to-seat contact point. This helps you narrow or move
the valve seat.
A few manufacturers do NOT
recommend valve lapping. Refer to the manufacturers
service manual for details.
Make sure you clean all of the valve grinding
compound off the valve and cylinder head.
The compound can cause rapid part wear.
Another way to check valve-to-seat contact is by
spreading a thin coat of prussian blue on the valve face
or putting lead pencil marks on the valve seat. If, when
turning the valve on its seat, an even deposit of coloring
Figure 3-63.Normal valve seat.
is seen on the valve seat or the pencil lines are removed,
the seating is perfect. The valve should NOT be rotated
more than one-eighth turn as a high spot could give a
false indication if turned one full revolution.
Figure 3-63 shows a normal valve seat. This will
vary according to the manufacturers specification. The
seat should touch near the center of the valve face with
the correct contact width. Typically, an intake valve
should have a valve-to-seat contact width of about 1/16
of an inch. An exhaust valve should have a valve-to-
seat contact width of approximately 3/32 of an inch.
Check the manufacturers service manual for exact
When the valve seat does NOT touch the valve face
properly (wrong width or location on the valve) (fig.
3-64), regrind the seat using different angles, usually
15-degree and 60-degree stones. This is known as
narrowing or positioning a valve (fig. 3-65).
To move the seat in and narrow it, grind the valve
seat with a 15-degree stone. This removes metal from
around the top of the seat. The seat face moves closer to
the valve stem.
To move the seat out and narrow it, grind the valve
seat with a 60-degree stone. This cuts away metal from
the inner edge of the seat. The seat contact point moves
toward the margin or outer edge of the valve.
Rocker Arm Service
After disassembling the rocker arms, you should
inspect them for wear, clogged oil holes, and damage.
Figure 3-64.Incorrect valve-to-seat contact.