month I'd like to get started on the bottom end of the GMC
engine. I am going to deal with the oiling system of these
There is no doubt that the GMC engine oiling system is one of
the best designed systems bar none. Much has been written about
how to get the most out of it with the least amount of effort.
Much of the great GMC racing success in the past can be
attributed to the fine design of this oiling system, it is very
simple and it flat works.
California Bill Fisher was the first to state in his speed
manual for the GMC, that a full flow modification to this engine
was like a low-cost insurance policy. I want to pass this on to
you in this column, the complete detailed procedure for this
modification. For an hour's time and less then a dollar, this
modification is a must for the street and of equal value for
racing on the dirt tracks.
I'm sure you guys all understand the benefits of filtered oil
in any type of internal combustion engine. Space here won't
allow me to get into these benefits now but it stands to reason
that every American engine produced in the last 25 years has an
oil filtering system from the factory. So the reasons are
obvious. One thing the GMC engine designers finally got wise to
is the filtering of this oil. Early GMC engines up to 1955 where
issued without any oil filtering device at all. They left this
to the detergent oil that was quite popular at the time. This by
today's standards, is quite archaic.
In addition, the 1946 Chrysler "full-flow" oil
filter adapted to the GMC motor is really a thing of the past.
These filter canisters can still be found, but the oil filter
element itself can be mighty scarce. Premium prices are usually
paid for these elements when and if they can be found. A more
simple and less expensive system can be set up with a few choice
pieces of readily available equipment on the market today.
Trans-Dapt. and Offenhauser offer a remote oil filter mounting
kit originally designed for engine swaps where the oil filter
can get in the way of steering and other obstructions.
remote adapter is really a time saver and when it comes to
changing oil filters it can be mounted anywhere you want. The
only restricting feature is the length of the oil lines to and
from the unit (by the way, it has 3/8 NPT outlets.) A super plus
feature of this remote adapter is that it takes readily
available spin-on filters. These new filters have very minimal
restrictions to the oil flow and a minimum oil pressure drop
through them. These adaptors also make changing oil filters a
fairly clean routine job. They are a much better plan of attack
on the oiling system than the '47 Chrysler units.
The accompanying drawing (figure 1) shows just
how easy this modification is. First, remove the pipe plug from
the existing 3/8 NPT hole in the side of the block. (A)
Secondly, thread the existing vertical hole (B) with a 9/16 X 18
tap. Insert a plug made from 9/16 X 18 cap screw, 1/2 inch long.
Saw a screwdriver slot for inserting this plug and Lock Tite
threads. The final step in this simple modification is to drill
and tap a hole in the side of the block for the outlet hose. Use
a 19/32" drill and tap for 3/8 NPT threads. Drill this hole
exactly 1 1/8" up from the bottom of the block. (C) There
you have it, low cost insurance indeed!