Onan NHC Series Operator's/Service Manual And Parts Catalog page 37

VALVES
Proper seated valves are essential-to good engine
performance. The cylinder head is removable for
valve servicing. Do not use a pry to loosen the
cylinder head. Rap sharply on the edge with a soft-
faced hammer, taking care not to break any cooling
fins. A.conventional type valve spring lifter may be
used when removing the valve spring locks, which are
of the split type.. Clean all carbon deposits from the
cylinder head, piston top, valves, guides, etc. If a valve
face is burned or warped, or the stem wom, install a
new valve.
Worn valve stem guides may be replaced from inside
the valve chamber. A seaMs provided behind the
intake valve guides only. The smaller diameter of the
tapered valve guides must face toward the valve head.
Tappets are also replaceable from the valve chamber,
after first removing the valve assemblies.
The valves should not be hand lapped, if at all
.avoidable, since the sharp contact may be destroyed.
This is especially important where stellite faced
valves and seats are used. Valve faces should be
finished in a machine to 44 degrees. Valve seats
should' be ground with a 45-degree stone and the
width of the seat band should be 1/32 to 3/64-of an
inch wide. Grind only enough to assure proper
seating.-
Remove all ..grinding compound from engine parts
and place each valve in its proper location. Check
each valve for a tight seat, using an air pressure type
testing tool. If such a tool is not available, make pencil
marks at intervals across the valve face and observe if-
the marks.rub off uniformly when the valve is rotated
part of a turn against the seat.
Lightly oil the valve stems and reassemble all parts
removed. Adjust the valve clearance.
Tha valve face .angle is 44 degrees. The valve seaf
angle is 45 degrees:This 1-degree interference angle
-results in a sharp seating surface between the valve
and the top of the.valve seat. The interference angle
method of grinding valves minimizes face deposits
and lengthens valve, life (Figure 51). -
The positive type valve rotocoils serve to prolong
valve life, and decrease valve repairs. Check the
rotocoils periodically by removing the cylinder heads
and cranking the engine. When functioningproperly,
the .valve is rotated a. fraction.of a-.turn each time it
opens. If rotocoils are faulty, install new ones.
/VOTTT-USE A STANDARD
AUTOMOTIVE TYPE ,
WRENCH TO ADJUST
THE TAPPETS. (&<
^ALVE SEAT"
^INSERT-4
NOTE: SEE VALVE TAPPET
isftCLEARANCES IN TEXT-,
V A L V E - ^
/
n ' -
7^
- S E A T
WRONG
[ V V A L V E
» SEAT
y /
^ f * . RIGHT
FIGURE 51. VALVE SYSTEM
35