short out the spark plugs one at a time. The noise
will be greatly reduced when the piston in the
cylinder that is responsible is not delivering
Piston-pin knock is identified more as a
metallic double-knock rather than a regular
clicking sound like that heard in valve and tappet
noise. In addition, it is most noticeable during idle
with the spark advanced. A check can be made
by idling the engine with the spark advanced and
then shorting out the spark plugs. Piston-pin noise
coming from a cylinder will be reduced somewhat
when the spark plug for that cylinder is shorted
out. Causes of this noise are a worn or loose
piston-pin, a worn bushing, and a lack of oil.
Piston-ring noise is also similar to valve and
tappet noise since it is identified by a clicking,
snapping, or rattling sound. This noise is most
noticeable on acceleration. Low-ring tension,
broken or worn rings, or worn cylinder walls will
produce this sound. To avoid confusing this sound
with other engine noise, make the following test:
remove the spark plugs and add an ounce or two
of heavy engine oil to each cylinder. Crank the
engine for several revolutions to work the oil
down past the rings. Replace the spark plugs and
start the engine, If the noise has decreased, it is
probable that the rings are at fault.
Piston slap may be detected by a hollow, bell-
like knock and is due to the rocking back and
forth of the piston in the cylinder. If the slap
occurs only when the engine is cold, it is probably
not serious. However, if it occurs under all
operating conditions, a further examination is
called for. The slap can be caused by worn
cylinder walls, worn pistons, collapsed piston
skirts, or misaligned connecting rods.
Crankshaft knock is a heavy, dull, metallic
knock that is noticeable when the engine is under
load or accelerating. When the noise is regular,
it can be contributed to worn main bearings.
When irregular and sharp, the noise is probably
due to worn thrust bearings.
In most shops, the Navy provides accurate and
dependable testing equipment. But having the
testing equipment in the shop is NOT enough. The
supervisor and the crew must know how to use
this equipment since proper use provides the
quickest and surest means of finding out what is
wrong and where the fault lies.
Four of the most widely used testing instru-
ments are the cylinder compression tester, vacuum
gauge, cylinder leakage tester, and tachometer.
As you have learned, engine power results
from igniting a combustible mixture that has been
compressed in the combustion chamber of an
engine cylinder. The tighter a given volume of fuel
mixture is squeezed in the cylinder before it is
ignited, the greater the power developed. Unless
approximately the same power is developed in
each cylinder, the engine will run unevenly. The
cylinder compression tester (fig. 3-9) is used to
measure cylinder pressure in psi, as the piston
moves to TDC on the compression stroke.
By measuring compression pressures of all
cylinders with a compression gauge, then
comparing them with each other and with the
manufacturers specifications for a new engine,
you get an accurate indication of engine condition.
The compression pressures in the different
cylinders of an engine may vary as much as 20
pounds. The variation is caused largely by the lack
of uniformity in the volume of the combustion
chamber. It is nearly impossible to make all the
combustion chambers in a cylinder head exactly
the same size. For example, in a given engine with
Figure 3-9.Cylinder compression tester.