Checking Valve Timing; Measuring Camshaft Lobe Lift; Camshaft Replacement - GMC G Service Manual

6A7-16 4.8 LITER L6

CHECKING VALVE TIMING

Tool Required:
J 8520 Dial Indicator Adapter
When it becomes necessary to make a check of valve
timing, the following procedure may be used:
1. Loosen the nut at the #1 intake valve rocker arm,
swing the rocker arm away from the pushrod, then
remove the pushrod.
2. Install a dial indicator with J 8520 (figure 26). Turn the
crankshaft until the #2 exhaust valve opens and the
notch on the damper is aligned with the "0" mark on
the timing tab.
3. Position the dial indicator to measure lifter movement
and set indicator at zero. Turn the crankshaft 360
degrees and read the indicator. On correctly-timed
engines the indicator will read 0.012-0.020-inch.
4. If the correct reading is not shown, reset the indicator
at zero and turn the crankshaft 360 degrees, then
read the indicator again. If reading is now within spec­
ifications, the engine is timed properly.
5. The following chart shows indicator readings with
gears properly indexed for 4.8L engines and the indi­
cator readings resulting from improperly indexed
gears.
Engine
Camshaft Part Number
Valve Lift
Dial Gears Properly Indexed
Indicator One Tooth Advanced
Readings One Tooth Retarded
n
Figure 26 — Checking Valve Timing Or Camshaft
Lobe Lift
MEASURING CAMSHAFT
LOBE LIFT
Tool Required:
J 8520 Dial Indicator Adaptor
4.8L
3848000
0.405"
0.012-0.020"
0.038"
0.007"
GMTB-0530-1L
J 8520
Use the following procedure to check for worn camshaft
lobes:
1. Remove the rocker arm. Refer to "Rocker Arm and
Pushrod Replacement" in this section.
2. Install a dial indicator with J 8520 (figure 26). Position
the dial indicator so the plunger rests on the pushrod
end, as shown. Make sure the pushrod is in the lifter
socket.
3. Rotate the crankshaft slowly in the direction of rota­
tion until the hydraulic lifter is on the heel of the cam
lobe. At this point the pushrod will be in its lowest
position. Set dial indicator on zero, then rotate the
crankshaft slowly until the pushrod is in the fully
raised position.
4. Compare the total lift recorded from the dial indicator
with specifications at the end of this section.
5. Continue to rotate the crankshaft until the indicator
reads zero. This will be a check on the accuracy of
the original indicator reading.
6. Remove the dial indicator and adapter J 8520.
7. Install the rocker arm. Refer to "Rocker Arm and
Pushrod Replacement" in this section.
I f f i Adjust
• Valves. Refer to "Valve Adjustment" in this section.

CAMSHAFT REPLACEMENT

To check for worn camshaft lobes without disassembling
the engine, refer to "Measuring Camshaft Lobe Lift" in this
section.
M Remove or Disconnect (Figures 27 and 28)
1. Front end sheet metal as required. Refer to SHEET
METAL (SEC. 2B).
• Drain the cooling system.
2. Radiator, fan shroud, cooling fan and pulley. Refer to
ENGINE COOLING (SEC. 6B1) and RADIATOR
(SEC. 6B2).
3. Air cleaner.
4. Fuel pump. Refer to FUEL SYSTEM (SEC. 6C).
5. Rocker arm cover. Refer to "Rocker Arm Cover
Replacement" in this section.
6. Distributor. Refer to ENGINE ELECTRICAL (SEC.
6D).
7. Pushrod covers. Refer to "Pushrod Cover Replace­
ment" in this section.
8. Hydraulic lifters. Refer to "Hydraulic Lifter Replace­
ment" in this section.
9. Torsional damper. Refer to "Torsional Dampener and
Front Crankshaft Seal Replacement" in this section.
10. Timing gear cover. Refer to "Timing Gear Cover
Replacement" in this section.
• Align the timing marks (figure 27).
11. Thrust plate bolts (93).
12. Camshaft. Support the camshaft carefully to avoid
damage to the camshaft bearings. On some vehicles,
it may be necessary to disconnect the front engine
mounting and raise the engine to provide clearance.