GMC C Series 1993, Repair Manual

Page: 181 / 264

V 8E N G IN E S , GASOLINE 6A2-25
• Valve stem clearance, as follows: .
1. Excessive valve stem clearance will cause
excessive oil consumption and may cause
valve breakage. Insufficient clearance will result
in noisy and sticky functioning of the valve and
disturb engine smoothness:
2. Clamp J 8001-M (or equivalent) on one side of
the cylinder head rocker arm cover gasket rail
(figure 49).
3. Locate the indicator so that movement of the
valve stem from side to side (crosswise to the
head) will cause a direct movement of the indi­
cator stem. The indicator stem must contact the
side of the valve stem just above the valve
guide bore.
4. Drop the valve head about 1.6 mm (1/16 inch)
off the valve seat.
; 5. Move the stem of the valve from side to side
using light pressure to obtain a clearance read­
ing. If clearance exceeds specifications, it will
• be
necessary to ream the valve guide bores for
oversize valves as outlined later.
• Valve spring tension using J 9666 (or equivalent)
(figure 50). .
— Compress the springs, with dampers removed,
to the specified height and check against the
specifications chart. Springs should be replaced
if not within 44 Newtons (10 lbs.) of the speci­
fied load
..
• Valve spring free length (figure 51). Compare with
“Specifications.”
VALVE GRINDING
Valves that are pitted must be refaced to the proper
angle. Valve stems that show excessive wear, or valves
that are warped excessively must be replaced. When a
valve head that is warped excessively is refaced, a
knife edge will be ground pn part or all of the valve
head due to the amount of metal that must be removed
to completely reface. Knife edges lead to breakage,
burning or pre-ignition 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, replace the valve
(figure 52).
Several different types of equipment are available for
refacing valves. The recommendation of the manufac­
turer of the equipment being used should be carefully
followed to attain proper results: The correct valve face
angle is 45 degrees.
V A L V E S E A T G R I N D I N G
, Reconditioning the valve seats is very important,
because the seating of the valves must be perfect for
the engine to deliver the power and performance built
into it.
Another important factor is the cooling of the valve
head. Good contact between each valve and its seat
will ensure that heat will be properly carried away.
: Several different types of equipment are available for
reseating valve seats. The recommendations of the
manufacturer of the equipment being used should be
carefully followed to attain proper results.
Regardless of what type of equipment is used, it is
essential that valve guide bores be free from carbon or
dirt to ensure proper Centering of the pilot in the guide.
The correct seat angle is 46 degrees.
R E A M I N G V A L V E G U I D E S
The valve guides used in engines covered by this
manual are pressed into the Cylinder head.
If the valve stem to bore clearance, as measured
previously in this manual, is excessive, the valve guides
should be reamed and a valve with an oversize stem
installed. Valves are available with 0.003 inch, 0.015
inch, and 0.030 inch oversize stems.
Select a reamer that will provide a straight, clean
bore through the guide's entire length (figure 53).
Reamer availability is as follows. Sizes given are
nominal. ' , .
• J 7049-1: 0.003 inch oversize
• J 7049-2: 0.015 inch oversize
• J 7049-3: 0.030 inch oversize
• These reamers are also available in Reamer
Set J 7049. \