Evening star., 1925-12-13

ings in reverse order. He fails To uUle
that the marks on the rod and it's cap
must match, and he does not discover
the situation until he has replaced
the crankcase, filled it up with fresl
oil, and tried to drive the car
Causes of Tapping.
Judging from the difficulty the ~
erage motorist has in silencing his
engine, II is evident that he appre-
ciates the fact that tapping can re
suit from too much clearance in the
tappet or valve assembly Excluding
the possibility Of a broken camshaf;
roller, q warped valve, worn valve
guides or a loose rocker arm in the
case of a valve-in head motor, the
simplest and most likely cause Of
valve assembly noise is the extra
stem Gn the l. or 7 head motor, or
the push rod and the end Of the valve
stem lin the l. or T-head motor. Or
between the end of the rocker arm and
the end of the valve stem Gn the valve
in-head motor
This being the case. the motorist
gets out the tool kit and proceeds t.
take up whatever clearance he finds
He knows that he must not work o.:
any tappet unless the piston of its re
spective cylinder is o upper dead
center. This is the point when both
valves Of a cylinder. intake and cx
haust, should be fully closed, and
when tappet clearance should be at its
maximum. He also knows he should
do the Job while the engine Is warm
| TO allow for valve stem expansion
But this does not save him from e..
| ing off on q tangent
Knowing SO much of the story Of
the valves, he feels justified in skip
ping over many of the details For
| gotten is a statement to the effect tha'
all valve tappet adjustments should
be tested for thigh spots" otherwise
| the remedy Is quite likely to be worse
than the trouble. Disregardng this
| entirely, he is az loss to understand
why his engine should tap worse afte.
| he cuts down the clearance.
| What really happens is that wher
the push rod turns around to a now
| position speaking first Of the L.head
| motor Of the "mushroom" tappe
| type). q high spot OF the cam fol
! lower begins TO ride the camshaft
with the result that the slight clear
| ance is entirely taken up. The valve
| stem expands. and. since there is ~
provision for such expansion, the
valve remains slightly open. This
causes a loud tapping and "A loss o.
| power, depending upon the amount of
additional lift caused By the "high
| spot"
Worse Mistakes Made.
In the valve in-head motor. cleaI-
ance is likely to be dissipated if the
rocker arm moves slightly to the right
or left, or if the surface of the end
| that comes in contact with the valve
stem end is not entirely smooth. Per
| haps the motorist makes the adjust
ment while a slightly worn part OF
the rocker arm touches the end of the
valve stem. When the arm shifts it@
position. a high spot" comes into
contact with the end of the valve
stem, and the valve IS held slightly
open as a consequence.
In either case. the nlotoriSt is puT-
| zled to know why his engine viii no
| he has read the story all wrong OF
| that the story itself is wrong. He con
| ceiveS the idea of using his own judg
| ment, and decides to adjust all tap
| pets with no clearance at all. Having
| done this, and finding that the engine
| will not run, he tries TO undo his work
| only TO find that he gets confused on
the valve timing.
| If Il motorist is going to profit BY
what he reads, il behooves him to pc
ruse each page carefully. There are
many angles to even the simplest OF
| car parts, and, unless the motorist
who likes to do his own repair woriz
| knows the whole story. he is likely I"
| find himself perpleXed when he runs
into a complication.
Copyright. 1920