Figure 3-19.Grinding valve seats using a concentric type
is rotated in the valve seat, The stone is kept
concentric with the valve guide by means of a self-
centering pilot (fig. 3-20), which is installed in the
guide. Check the self-centering pilot for trueness
before using, A damaged pilot will cause the seat
position to move in relation to the valve guide.
The valve guide must be kept clean and in good
condition. Most of the concentric grinders of the
Navy automatically lift the stone off the valve seat
about once every revolution to allow the stone to
clean itself of dust and grit by centrifugal action.
The abrasive stone must be dressed frequently
with a diamond-tipped dressing tool, such as that
shown in figure 3-21. Dressing the stone will
ensure a uniform, even grinding of the valve seat.
After the seat is ground, it will be too wide.
To narrow it, use upper and lower grinding stones
to grind away the upper and lower edges of the
seat. Figure 3-22 shows a typical valve seat that
was ground at 45°, then narrowed at the top with
Figure 3-20.Self-centering pilot.
Figure 3-21.Stone dresser.
Figure 3-22.Valve contact correction.
a 20° grinding stone, and then ground at the
bottom with a 70° grinding stone to narrow and
center the valve seat.
To test the contact between the valve seat and
the valve, mark lines with a soft pencil about one-
fourth inch apart around the entire face of the
valve. Next, put the valve in place and rotate,