Onan NH User Manual

CYLINDER HEAD STUD TEST
AND REPLACEMENT PROCEDURE
This cylinder head stud replacement procedure
should be used whenever replacing any of the top six
studs on a NHC block. The use of a Helicoil is not a
recommended repair procedure. If a Helicoil has
been used, thecylinder block must be replaced. Gra-
phoil head and intake manifold gaskets should also
be used when replacing cylinder head studs.
Parts Required
Part No. Description
520-091 2 Step stud
11 0-2987 Head gasket
154-221 9 Intake manifold gasket
420-0398 Drilling fixture (Reusable tool)
HEATTREATED
d
HARDENED
WASHERS
HEAD
GASKET
-
6 LONG STUDS (TOP)
4 SHORT STUDS (BOTTOM)
1. Disconnect the spark plug wires and remove the
spark plugs and cylinder head air shrouds from TORQUETOP SIX NUTS
TO
12 LB-FT. (COLD)
TORQUE BOTTOM FOUR
NUTS TO 15 LB-FT.
each cylinder.
2. Remove the nuts and compression washers (do not
remove the flatwashers) from the top six studs on
each cylinder head (Figure 80). Each stud will have
two compression washers and one flatwasher
arranged in the sequence shown in Figure 81.
Do not remove the nuts from the bottom four studs
before the test procedure is completed.
USE STUDS SHOWN ABOVE LINE
FOR SECTION 1 STUD TEST
LEFT CYLINDER RIGHT CYLINDER
NUMBERS INDICATE CORRECT
TIGHTENING SEQUENCE FOR
CYLINDER HEAD NUTS
FIGURE 80. CYLINDER HEADS
FIGURE 81. CYLINDER HEAD STUDS
3. Replace the nuts and then test the top six cylinder
head studs by tightening (use an accurate torque
wrench) each nut to
30 ft-lb (40 Nm) of torque.
Make a note of any studs that cannot be tightened
to
30 ft-lb of torque.
4. Remove the cylinder-head nuts, flatwashers,
cylinder head, and head gasket. Discard the head
gasket and remove any studs that could not be
torqued to 30 ft-lb (40 Nm) without thread pull
out.
5. Examine the gasket surfaces of the block and
cylinder head for distortion or irregularities that
could cause leakage. Check the head and block
for warpage by laying a straight edge over the
length of the gasket surface.
If a 0.005 inch feeler
gauge fits between the straight edge and gasket
surface, the part must be replaced or milled flat.
A
maximum of 0.010 inch may be machined from
the cylinder block or head.
1
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Onan NH Specifications

General

Brand	Onan
Model	NH
Category	Engine
Language	English