Chevrolet 10 Series 1968, Service Manual

Page: 377 / 713

ENGINE 6-30
d. Crank the engine on revolution until the pointer
“ O” mark and torsional damper mark are again
in alignment. This is number 6 firing position.
With the engine in this position the following
valves may be adjusted.
Exhaust - 2, 5, 6, 7
Intake - 3, 4, 6, 8
4. Install rocker arm covers as outlined.
5. Adjust carburetor idle speed and mixture.
Valve Lifters
Hydraulic valve lifters very seldom require attention.
The lifters are extremely simple in design, readjust­
ments are not necessary, and servicing of the lifters
requires only that care and cleanliness by exercised in
the handling of parts.
Locating N oisy Lifters
Locate a noisy valve lifter by using a piece of garden
hose approximately four feet in length. Place one end
of the hose near the end of each intake and exhaust valve
with the other end of the hose to the ear. In this manner,
the sound is localized making it easy to determine which
lifter is at fault.
Another method is to place a finger on the face of the
valve spring retainer. If the lifter is not functioning
properly, a distinct shock w ill be felt when the valve
returns to its seat.
The general types of valve lifter noise are as follows:
1. Hard Rapping Noise—Usually caused by the plunger
becoming tight in the bore of the lifter body to such
an extent that the return spring can no longer push
the plunger back up to working position. Probable
causes are:
a. Excessive varnish or carbon deposit causing
abnormal stickiness.
b. Galling or “ pick-up between plunger and bore of
lifter body, usually caused by an abrasive piece
of dirt or metal wedging between plunger and
lifter body.
2. Moderate Rapping Noise—Probable causes are:
a. Excessively high leakdown rate.
b. Leaky check valve seat.
c. Improper adjustment.
3. General Noise Throughout the Valve Train—This
will, in almost all cases, be a definite indication of
insufficient oil supply, or improper adjustment.
4. Intermittent Clicking—Probable causes are:
a. A miscroscopic piece of dirt momentarily caught
between ball seat and check valve ball.
b. In rare cases, the ball itself may be out-of-round
or have a flat spot.
c. Improper adjustment.
In most cases where noise exists in one or more lifters
all lifter units should be removed, disassembled, cleaned
in a solvent, reassembled, and reinstalled in the engine.
If dirt, corrosion, carbon, etc. is shown to exist in one
unit, it more likely exists in all the units, thus it would
only be a matter of time before all lifters caused trouble.
Removal
1. Remove intake manifold as outlined.
2. Remove valve mechanism as outlined.
3. Remove valve lifters.
NOTE: Place valve lifters in a rack so they
may be reinstalled in the same location.
Installation
1. Install valve lifters.
NOTE: Whenever new valve lifters are being
installed coat foot of valve lifters with Molykote
or its equivalent.
2. Install intake manifold as outlined.
3. Install and adjust valve mechanism as outlined.
Valve Stem Oil Seal and/or Valve Spring
Replacement
1. Remove rocker arm cover as outlined.
2. Remove spark plug, rocker arm and push rod on the
cylinder(s) to be serviced.
3. Apply compressed air to the spark plug hole to hold
the valves in place.
NOTE: A tool to apply air to the cylinder is
available through local jobbers or may be fab­
ricated using an AC-46N Spark Plug or its
equivalent. This will make the tool universal for
all Chevrolet engines. Chisel the spark plug as
shown, then drive the porcelain out of the plug
by tapping the center electrode against a hard
block. Using a 3/8" pipe tap, cut threads in the
remaining portion of the spark plug and assem­
ble as shown (fig. 4V).
4. Using Tool J-5892 to compress the valve spring,
remove the valve locks, valve cap (or rotator) and
valve spring and damper (fig. 5V).
5. Remove the valve stem oil seal.
6. Assemble as follows:
307 and 327 cu. in. Engines
a. Set the valve spring and damper, valve shield and
valve cap (or rotator) in place. The close coiled
end of the spring is installed against the cylinder
head. Compress the spring with Tool J-5892 and
install oil seal in the lower groove of the stem,
making sure the seal is flat and not twisted.
NOTE: A light coat of oil on the seal will help
prevent twisting.
iin... _____________ b. Install the valve locks and release the com­
pressor tool making sure the locks seat properly
F ig . 4V— A ir Adapter Tool in the upper groove of the valve stem.
CHEVROLET TRUCK SERVICE MANUAL