Evening star. [volume] (Washington, D.C.) 1854-1972, 1925-02-01
No. 83-"Diagnosing Knocks.

Smith had worried for quite some
time over an annoying knock in the
motor but when | next met him he
was plainly puzzled, for he had been
getting diagnoses from various me-
chanics and found that they all diff
fered.
"One tells me it's a loose connect
ing rod bearing. Another says the
cylinders are out of round. A third
repairer declares a loose piston pin, y
Smith complained. How am l to
know what to believer
Get all the opinions on the subject
that you can and then use your own
judgment as to which diagnosis
sounds most plausible" l suggested.
The IDEAS all right" he agreed,
but I'm not a mechanic"
You don't have to be mechanic
to diagnose knocks" ] said, remind
ing him that many automobile me-
chanics do not clearly understand the
principles of the gas engine, though
they may be skillful with tools. You
wouldn't have to know much about
an engine to prove that the diagnosis
about the loose connecting rod bear
ing was wrong. You know that the
function of a connecting rod is to
link its piston with the respective
crank of the engine shaft. If the
bearing which joins the lower end
of the rod to the crank is loose it
will knock regularly and at all
speeds"
But that isn't the nature of the
trouble, Smith explained. The
knock ! hear begins at about %
miles an hour and varies with diff
ferent speeds, sometimes disappear
ing altogether"
Loose Bearing illuminated"
Then that eliminates the loose
bearing theory altogether AS for the
cylinders being out of round worn.
YOU know that can't be the case,
because your motor has plenty Of
power and is very economical on
gas and Oil A worn cylinder always
wastes fuel and OIL
That theory being off the list we
proceeded to consider the possibility
of a loose piston pin. This l prompt
ly outlawed by reminding him that
this type of trouble would also be
characterized by regularity and that
1t would be of a light, sharp nature"
How is the average car owner to
make such decision when he doesn't
even know what piston pin is?'
Smith asked.
"A hasty inspection of the diagram
of your engine in the instruction book
will show you that the piston pins
join the upper ends of the connect
ing rods to the pistons," / explained.
But you would not even have to
know this. If one mechanic you con-
suit diagnoses the trouble as a loose
piston pin you ought to ask each
one of the other mechanics il this
could be the case. In telling you
why the other mechanic guessed
wrong they will be unconsciously ex-
planning to you the function and
general characteristics of the piston
pin. In this way you not only find
but also how much he doesn't know
which is important. By obtaining
number of opinions you can run
them through a sort of clearing house
and come close to hitting the trouble
on the head.
The consensus of opinion will be
that the knock you hear in the mo-
tor is neither heavy enough to be
bearing trouble nor light enough to
be a worn piston pin. The logical
thing to do then is to look for some
trouble with the pistons, since they
are the only other movable parts
in the cylinders which are capa Bile
of making such a pronounced noise.
Your valves and the tappets. push
rods and other valve mechanism will
frequently produce odd noises, but
they will be light and will fluctuate