GMC T Series 1994 Repair Manual page 540

272
A. NOTCH (LOCATION MAY VARY.)
272. PRECHAMBER
Figure 54-—Prechamber
5. Prechambers (272) (if required) (figure 54).
• Drive out with a small nylon or brass drift insert­
ed through the injection nozzle hole.
• The light and heavy duty naturally aspirated
engines along with the light duty turbocharged
engine have prechambers that are constructed
of high temperature super alloy material. Their
throat and internal configuration is new. The
dimensions of the prechambers are the same.
The heavy duty turbocharged engine uses the
same high temperature super alloy material but
its throat size is larger and its internal dimen­
sions are different than the light and heavy duty
naturally aspirated engines and light turbo­
charged engine.
Cleaning and Inspection of Components
Components as outlined in SECTION 6A.
1 ?
Inspect (Figure 55)
1. Components outlined in SECTION 6A. Replace all
valve springs at overhaul.
2. Prechambers for cracks (figure 55). Replace any
prechambers with facial cracks longer than 5 mm
(3/16 inch). Service prechambers are available in
standard and 0.254-mm oversize.
Measure (Figure 56)
1. Valve stem clearance. Refer to SECTION 6A.
2. Cylinder head warpage. Use a straightedge and a
feeler gage (figure 56). If the head is warped more
than 0 .15 -m m (0 .0 0 6 -in c h ) lo n g itu d in a lly or
0.08-mm (0. 003-inch) transversely, replace the cyl­
inder head. Resurfacing is not recommended due
to the extremely close valve to piston clearances.
3. Cylinder head thickness (rocker arm cover gasket
rail to head gasket surface) must be at least 97.87
mm (3.853 inch).
ACCEPTABLE
CRACK LENGTH
NOT OVER
5 mm (3 /ie")
B8413
A. Head Gasket Sealing Bead Mark
B. Fire Slot
Figure 55— Prechamber Inspection
A. Straight Edge
B. Feeler Gage
Figure 56— Checking the Cylinder Head for
4. Valve protrusion. This is the distance the valve is
extended past or recessed into the firedeck. If the
valve is past specification (recessed too far) the
pushrod can bottom out in the lifter and hold the
valve open. Spring seat tension will be lost, as well
as compression. To check the valve protrusion use
J 4209-25 sled gage, with plunger resting on the
surface of the firedeck. Set gage to 0 then move
the plunger over each valve and note the reading
on the gage. This measurement will be the amount
of valve protrusion, Maximum protrusion is -1.143
mm (0.045 inches) for intake and - 0.889 mm
(0.035 inches) for exhaust. The
valve is recessed into the head.
UNACCEPTABLE
CRACK LENGTH
OVER 5 mm
p h e ")
B8399
B8423
Warpage
denotes that the