Compression of a standard new engine
prior
to
Spec
P at
about 300rpm is approximately 300 - 350
psi.
Beginning
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
indi-
cation
of leakage is a compression increase when oil is
added to the cylinder.
DISASSEMBLY
1.
Remove the decompression solenoid.
2..
Remove the rocker box cover,
fuel
nozzle and connect-
ing oil
lines to the cylinderhead.
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 as-
sembly.
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° angle and exhaust valves to a 45°
angle. But, if they are badly
pitted,
or
will
have a
thin
edge
when refacing, replace them.
Check refinished valves toi a
tight
seat
to the valve
seat
with
an air
pressure
type testing
tool
or by applying Prussian
Blue
on the valve face and rotating it against the
seat.
Valve
Guides: Check valve guide to valve clearance, see
Table of Clearances. 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"
from
the rocker box
side of the head. Ream the new valve guide to obtain the
proper clearance.
Volve
Seats:
if the valve
seats
are
pitted,
refinish
them.
Using
conventional
seat
grinding
equipment, reface each
seat
to a 45° angle and a
seat
width
of 3/64 to
1/16".
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
#4208272 in a
drill
press
(Fig. 11-3) to
remove each valve
seat.
Adjust
the
tool
to cut 1/64"
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 .002", .005", .010" and .025". Otherwise,
install
new standard size
seat
inserts.
Drive
the new valve
seat
inserts
into
place. Be certain that
each
seat
rests
solidly
on the bottom of the counterbore 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° angle and
width
of approxi-
mately
3/64". 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 .030". If
it
is not, regrind the
seat.
Valve
Springs: Check the valve springs on an accurate com-
pression scale.
Valve
spring data is given in the Table of
Clearances. 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 surface of the
seal.
31
DRILL
PRESS
Be
sure
to
oil
pilot
of
tool.
After
cutting,
rind
is easily
MAKE
CUT 1/64"
FROM
EDGE
OF
SEAT.
FIG.
11-3
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