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Sec. 4
Page 70
GM COACH MAINTENANCE MANUAL
AIR COMPRESSOR AND GOVERNOR
UNLOADER
UNLOADER
UNLOADER
VALVE SP
VALVE
VALVE STOP
VALVE SPRING
RETAINER
TP 1826
Figure 24-lnstalling Unloading Valves
the other side, as this will destroy the value of
the punch marks on the nuts.
1. Position connecting rod in piston and press
piston pin into piston, making sure the piston pin
lock hole lines up with the lock hole in the piston.
Install new piston pin lock, making sure the lock
engages holes in piston pin and piston.
2. Install piston rings in grooves in pistons.
Four rings are used on each piston. Compression
rings must be installed with silver mark on edge
of ring up. Rings must be installed in their prop-
er location and position as shown in figure 22.
Note that segment rings and expander rings are
used in conjunction with oil ring and scraper ring.
3.
Press bearing inserts into rod and cap by
hand, making sure locking slots are aligned.
4.
Lubricate pistons, rings, piston pin bush-
ings, and bearing inserts with clean engine oil.
5. Turn crankshaft to position No. 1 bearing
journal downward.
Remove bearing cap from No.
1 connecting rod, leaving bolts in rod.
SCREW DRIVER'-.._,__
""'1
UNLOAD_E_~_
~
VAL VE LEVER
I
LOCK NUT
UNLOADER
VALVE
SPRING
DISCHAR
PORT
Figure 25-Adjusting Unloading Valves
6. Insert No. 1 connecting rod and piston
through top of No. 1 cylinder, being sure the con-
necting
roe!
bearing engages the crankshaft journal
properly.
Install bearing cap and nuts, referring
to note at beginning of this procedure. After nuts
are properly installed and tightened, install new
cotter pins.
7. Install No. 2 piston and connecting rod in
same manner as described above.
8.
Position crankcase bottom cover and new
gasket on crankcase and attach with six cap screws
and lock washers.
CYLINDER HEAD ASSEMBLY AND
INSTALLATION
1.
Insert each unloading valve into cylinder
head body from bottom side. Place a small wood-
en block or a nut under each valve to hold it up
against seat as head is set on bench. Install un-
loading valve spring over each valve, then place
stop on top of each spring.
Compress each un-
loading valve spring by hand and install spring
retainer on valve stem above stop (fig. 24).
2.
Place each discharge valve on its seat
through opening in top of cylinder head.
Place
discharge valve springs in discharge valve cap
nuts, then thread cap nuts into cylinder head.
Tighten nuts firmly.
3. Lubricate unloading diaphragms with a thin
coating of light engine oil and place diaphragms
in cavity in top of cylinder head. Place diaphragm
follower in position on diaphragms with post up-
ward.
Install diaphragm cover over diaphragm
follower post and attach cover to cylinder head
with screws and lock washers.
Tighten screws.
4. Check forleakage past unloading diaphragms
by applying
100
pounds air pressure to the unload-
ing diaphragm cavity port and apply soap suds all
over the diaphragm cover, particularly around the
diaphragm follower post.
Leakage in excess of a
1-inch bubble in three seconds necessitates dis-
assembly and correction of leakage.
5.
Place dust guard in position over end of
diaphragm follower post.
6. Install adjusting screws in unloading valve
lever with the screw heads and nuts on the under
side of the lever.
7.
Place unloading valve lever spring in re-
cess in diaphragm cover and position lever over
spring. Insert lever pin through diaphragm cover
and lever and install a cotter pin in each e~d of
pin.
8. Install drain plug in bottom of cylinder
head
if
it has been removed.
9. Install a new cylinder head gasket over
studs so the cut-outs in the gasket will register
with the unloading valves when the cylinder head
is installed.
Install cylinder head on cylinder
block so the air discharge port will be on the
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