Onan MJA Series Service Manual page 77

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2. . Re.move the rocker box cover, spark plugs and connecting oil and
water lines to the cylinder heads.
3. Remove the intake and exhaust manifold.
4. Remove the capscrews holding each cylinder head to the cylinder
block.
5. Remove each head.
If
it sticks, rap it sharply with a soft hammer.
Don't use a pry.
6. Remove the rocker arms a.:ud pushrods.
7.
U~ing
a valve
Spring...c;.;r~1.~.:/'~~,;,~~s.
assemble the valve assem ­
blles.
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3.7.4 REPAIR
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Thoroughly clean all compoh-ents
.fi1
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1-Be cylinder head assemblies.
Remove all the carbon deposits from the combustion chambers and
Clean all gasket surfaces.
3.7.4.1 Valves. - Remove all carbon and check each valve for burn ­
ing, pitting. or warped stem. Valves that are. slightly pitted orburned,
refinish on an accurate valve grinder to a 45
0
angle. But, if they are
badly pitted, or will have a thin edge when refaced, replace them.
Check refinished valves for a tight seat to the valve seat with an air
pres~;ure
type testing tool or by applying Prussian Blue on the valve faCE
and rotating it against the seat.
3.7.4.2 Valve Guides'5t...,.:.. Check valve-guide-to-valve clearance, Sect
9.
If the proper clearances
can't·p~\.o,htained
by replacing the valves, re ­
place the valve. guides.
DriV'€'~\\~'old
valve guides into the valve cham ­
bers. Drive new guides in ufit$t}ihey protrude 11/32" from~he rocker
. box. side of the head. Ream the new valve guide to obtain the proper
clearance, Sect 9.
3.7.4.3 Valve Seats. - If the valve seats are pitted, refinish them.
Using conventional seat grinding equipment, reface each seat
to
a 45
0
angle and a seat width of 3/64 to 1/16 inch. You should be able to
reface each seat several times before it becomes necessary to replace
If,
however, the valve seats are loose or cannot be refaced, replace
them.
Use ONAN tool #420A272 ina
drill
press (Fig 3.7.0) to remove each
valve seat. Adjust the tool to cut 1/64 inch from the edge of the seat.
CYLINDER HEADS AND VALVES