Chevrolet 1988 Light Duty Truck Repair Manual page 348

6A-18 GENERAL ENGINE MECHANICAL
refa ce . K nife e d g e s lead to b re a ka g e , 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 equipm ent are available for
refacing valves. The recom m endation of the manufacturer
of the equipm ent being used should be carefully followed
to attain proper results.
Refer to " S pecifications" in the proper section for valve
face angle specifications.
VALVE SEAT GRINDING
R e co n ditio n in g the valve seats is very im p o rta nt,
because the seating of the valves must be perfect for the
engine to deliver the power and perform ance 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 re co m m e n d a tio n s of the
manufacturer of the equipm ent being used should be
carefully followed to attain proper results.
Figure 3 5 — Valve Head Measurement
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 "'S pecifications" in the proper section for valve
seat angle specifications.
NOTICE: Valve seats on 2.5 L a n d 6.2 L en g ine s
are in d u c tio n hardene d. E xcessive rem oval o f
s to c k m ay re s u lt in dam age to the valve seats.
4.8 L, sm all b lo c k , a nd 7.4 L e n g in e s are e q u ip p e d
w ith harde ned , n o n -re p la c e a b le valve seat inse rts.
REAMING VALVE GUIDES
The valve guides used in engines covered by this
manual are sim ply holes bores into the cylinder head. The
valve guides are therefore not replaceable.
If the valve stem to bore cle a ra n ce as m easured
previously in this manual is excessive, the valve guides
should be reamed and a valve with an oversize stem
installed.
A vailable oversize valves are as follows. N om inal
dim ensions 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).
— E xhau st: 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).
R eam er a va ila b ility is as follow s. 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 ssem b le
*
Valves and components. Refer to the proper section.