Onan DJA Series User Manual

Compression of a standard new engine prior to Spec P
at about 300rpm is approximately 300 - 350psi. Begin-
ning Spec P, compression is about 350 - 400 psi.
Compression reading will deviate considerably from the
above readings because of differences in cranking
speed, altitude and ambient temperature conditions.
Therefore the specification is given only as a guide.
The best indication of leakage is a compression in-
crease when oil is added to the cylinder.
Disassembly:
1. Remove the decompression solenoid.
2. Remove the rocker box cover, fuel nozzle and
connecting oil lines to the cylinder head.
3. Remove the intake and exhaust manifold.
4. Remove the cap screws holding the cylinder head
to the cylinder block .
5. Remove the head. If it sticks, rap it sharply with
a soft hammer. Do not use a pry.
6. Remove the rocker arms and push rods.
7. Using a valve spring compressor, disassemble the
valve assemblies.
Repair:
Thoroughly clean all components of the cylinder head
assembly. Remove all the carbon deposits from the
intake and exhaust ports and clean all gasket surfaces.
Valves: Remove all carbon and check each valve for
burning, pitting, or warped stem. Valves that are
slightly pitted or burned, refinish on an accurate valve
grinder. Refinish intake valves to a 42-degree angle
and exhaust valves to a 45-degree angle. But, if they
are badly pitted, or will have a thin edge when refacing,
replace them.
Check refinished valves for a tight seat to the valve
seat with an air pressure type testing tool or by apply-
ing Prussian Blue on the valve face and rotating it
against the seat.
Valve Guides: Check valve guide to valve clearance,
see Dimensions and Clearances Section. If the proper
clearances cannot be obtained by replacing the valves,
replace the valve guides. Drive the old valve guides
into the valve chambers. Drive new guides in until
they protrude 11/32-inch from the rocker box side of
the head. Ream the new valve guide to obtain the pro-
per clearance.
Valve Seats: If the valve seats are pitted, refinish
them.. Using conventional seat grinding equipment,
reface each seat to a 45-degree 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 it.
If, however, the valve seats are loose or cannot be
refaced,, replace them. Use Onan tool No. 420B272 in
a drill press (Figure 39) to remove each valve seat.
Adjust the tool to cut 1/64-inch from the edge of the
seat. Oil the pilot to prevent it from seizing in the
valve guide. Cut each seat down to a narrow rind on
edges and bottom and break it out with a sharp tool.
Be careful not to cut into the counterbore bottom.
Thoroughly clean the valve seat counterbore and remove
any burrs from the edges. If the counterbore is damaged,
it will have to be remachined for an oversize seat.
Oversize seats are available in 0.002-inch, 0.005-inch,
0. 010-inch and 0.025-inch. Otherwise, install new stan-
dard size seat inserts.
Drive the new valve seat inserts into place. Be certain
that each seat rests solidly on the bottom of the coun-
terbore at all points. To make installation easier, heat
the cylinder head in an oven at 325
0
F for about 1/2-
hour and cool the valve seats in dry ice.
Face each new seat to a 45-degree angle and width of
approximately 3/64-inch. The finished seat face should
contact approximately center of the valve face. Use
Prussian Blue on each valve face to check this. Make
any corrections on the seat, not the valve face.
When the new seats are installed and faced, insert the
valve into each and check the clearance from valve
head to the face of the cylinder head. This must be at
least 0.030-inch. If it is not, regrind the seat.
Valve Springs: Check the valve springs on an accurate
compression scale. Valve spring data is given in the
Dimensions and Clearances Section. Replace any
spring that is weak, cracked or pitted or has ends out
of square.
Installation:
1. Push a valve stem oil seal onto the intake valve
guide and clamp in place. Then oil the inside
of the seal.
IMPORTANT: Units built before June 1962 had
no valve seals.
DRILL
PRESS
Be sure to
oil pilot of
tool..
After cutting,
rind is easily
removed.
MAKE CUT 1/64?'
FROM EDGE OF
SEAT.
FIGURE 39. REMOVING VALVE SEATS
36

Onan DJA Series Specifications

General

Brand	Onan
Model	DJA Series
Category	Engine
Language	English