GMC 1976 ZEO 6083 Maintenance Manual page 286

Worn or loose piston pin or bushing.
Improper fit of pin.
(Listen for a light ticking or tapping
noise. More noticeable with no load on engine .
May disappear completely under load .
Generally piston pin noise can be noticed on
deceleration of the engine .)
Piston-to-cylinder bore clearance ex-
cessive .
(Sounds very similar to tappet or lifter
noise. Removing one spark plug wire at a time
may be helpful in determining which cylinder
is noisy. One indication of piston slap is a
decrease in noise as the engine warms up .
Piston slap is always louder when the engine
is cold . Retard timing slightly, noise should
decrease .)
Lack of lubrication .
Carbon deposits on top of piston strikes
cylinder head .
Worn or broken piston ring land . (Most
noticeable during acceleration .)
GENERAL
E . NOISY PISTONS
NOTE: Piston pin, piston and connecting rod noises are difficult to tell apart. A
loose piston pin causes a sharp double knock which is usually heard when engine
is idling . Severity of knock should increase when spark plug to this cylinder is
short-circuited . However, on some engines the knock becomes more noticeable
at 25 to 35 mph on the rod .
GENERAL
F. NOISY VALVE MECHANISM
MECHANICAL
Sticking or warped valves .
Bent push rods .
Dirty, worn, or scored parts .
Broken or weak springs.
Damaged valve lifter and/or camshaft lobes.
Insufficient or poor oil to valve mecha-
nism . (Thin, foaming, or diluted.)
Excessive valve stem-to-guide clearance.
Valve lifter incorrectly fitted to bore
size .
Pulled or loose rocker arm bolts.
Broken or cracked piston .
Engine overheating.
Fuel of too low octane rating .
Operating without air cleaner.
MECHANICAL
ENGINE 6A- 2 1
Excessive rod bearing clearance.
Misaligned connecting rods .
Worn rings, cylinder walls, low ring
tension, broken rings, out-of-round or
tapered bores.
Top of piston strikes ridge at top of
cylinder bore .
Piston rubs against cylinder head gasket .
Excessive side clearance of rings in
groove, clearance between ring and groove
and/or ring gap.
Undersize pistons installed .
Wrong type and/or size rings installed .
Cylinder bores tapered or eccentric.
Pins improperly assembled.
Insufficient ring gap clearance.
Pistons 180° out of position .
Hydraulic lifter not working properly or
faulty . (Faulty lifter can usually be located
with the aid of a stethoscope.)
Hydraulic lifter :ipumped up" from ex-
cessive speed-temporary noise.