Valve Seat Service
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 service.
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 manufacturer’s manual for
specific.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 wiggle.
- 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-andsleeve 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
good seat.
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 surfaces.
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-toseat contact point. This helps you narrow or move
the valve seat. A few manufacturers do not recommend valve lapping. Refer to the
manufacturer’s service manual for details.
WARNING
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 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 manufacturer’s 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
manufacturer’s service manual for exact values.
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. |
