GMC 1989 Light Duty Truck Repair Manual page 352

6A-18 GENERAL ENGINE MECHANICAL
Figure 34— Measuring Valve Spring Length
refa ce. Knife e dges lead to breakage, b u rn in g or
preignition due to heat localizing on this knife edge. If the
edge of the valve head is less than 0.80 mm (1 /32-inch)
after grinding (figure 35), replace the valve.
Several different types of equipment are available for
refacing valves. The recommendation of the manufacturer
of the equipment being used should be carefully followed
to attain proper results.
Refer to " Specifications" in the proper section for valve
face angle specifications.
VALVE SEAT GRINDING
R econditioning the valve seats is very im portant,
because the seating of the valves must be perfect for the
engine to deliver the power and performance built into it.
Another inportant factor is the cooling of the valve
head. Good contact between each valve and its seat will
insure that heat will be properly carried away.
Several different types of equipm ent are available for
reseating valve seats. The recom m endations of the
manufacturer of the equipm ent being used should be
carefully followed to attain proper results.
Figure 3 5 — Valve Head M easurem ent
Regardless of what type of equipm ent is used, however,
it is essential that valve guide bores be free from carbon
or dirt to ensure proper centering of the pilot in the guide.
Refer to "'Specifications" in the proper section for valve
seat angle specifications.
NOTICE: Valve seats on 2 .5 L an d 6 .2 L engines
are in d u ctio n h ard ened . Excessive rem oval o f
stock m ay resu lt in dam age to the valve seats.
4 .8 L, sm all block, and 7.4 L engines are e q u ip p ed
with hardened, n on -replaceab le valve seat inserts.
REAMING VALVE GUIDES
The valve guides used in engines covered by this
manual are simply holes bores into the cylinder head. The
valve guides are therefore not replaceable.
If the valve stem to bore clearance as m easured
previously in this manual is excessive, the valve guides
should be reamed and a valve with an oversize stem
installed.
Available oversize valves are as follows. Nom inal
dimensions are given.
— 2.5 L:
— In ta k e : 0.08 mm (0 .0 0 3 -in c h ), 0.13 mm
(0.005-inch).
— Exhaust: 0.08 mm (0.003-inch).
— 2.8 L, 4.8 L, 5.0 L, 5.7 L, 7.4 L:
— Intake and exhaust: 0.08 mm (0.003-inch),
0.38 mm (0.015-inch), 0.76 mm (0.030-inch).
— 4.3 L:
— In ta k e : 0.08 mm (0 .0 0 3 -in c h ), 0.38 mm
(0.015-inch), 0.76 mm (0.030-inch).
— Exhaust: 0.08 mm (0.003-inch), 0.38 mm
(0.015-inch).
— 6.2 L:
— Intake and exhaust: 0.08 mm (0.003-inch), 0.38
mm (0.015-inch).
Select a reamer which will provide a straight, clean
bore through the guide's entire length (figure 36).
Ream er availability is as follows. Sizes given are
nominal.
• All engines except 6.2 L and 7.4 L:
— J 5830-1: 0.08 mm (0.003-inch) oversize.
— J 6621: 0.13 mm (0.005-inch) oversize.
— J 5830-2: 0.38 mm (0.015-inch) oversize.
— J 5830-3: 0.76 mm (0.030-inch) oversize.
— These reamers (except J 6621) also available
in Reamer Set J 5830-02.
• 6.2 L and 7.4 L engines:
— J 7049-1: 0.08 mm (0.003-inch) oversize.
— J 7049-2: 0.38 mm (0.015-inch) oversize.
— J 7049-3: 0.76 mm (0.030-inch) oversize.
— These reamers also available in Reamer Set
J 7049.
ASSEMBLY
*
A s s e m b le
Valves and components. Refer to the proper section.