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neither can really be called better than the other. The important
thing to remember is that neither type can protect your engine
from dirty oil if you don’t regularly service and maintain it.
A full-fl ow type fi lters all the engine oil each time it makes
a circuit through the lubricating system. Some might say this is
better, since all the oil is fi ltered before it gets pumped to the
bearings.
The problem is that, since all the oil must pass through a full-
fl ow fi lter each time it goes around, the fi lter element cannot be
restrictive. It cannot slow down the fl ow of oil.
Therefore, the fi ltering media of a full-fl ow fi lter cannot be
made as fi ne as a bypass type. It can’t fi lter out very small par-
ticles of dirt and grit.
In addition, most engines with full-fl ow fi lters have a by-pass
valve, either as part of the fi lter body or incorporated into the
spin-on element. This valve is calibrated to open if the fi lter be-
comes clogged. This allows oil to bypass the fi lter media and
circulate through the engine as if there was no fi lter at all.
THE BYPASS FILTER
A bypass fi lter cleans only a portion of the oil each time it
makes a circuit through the engine. Therefore, it can be made
fi ner to trap smaller particles of dirt, grit, and metal.
From late 1930s up into the mid-1960s, there was a prolifera-
tion of aftermarket bypass oil fi lters. Just about every company
that made fi lter elements also offered cases to put them in. The
result that was a bewildering array of fi lter choices. Unfortu-
nately, for the owners of vintage vehicles today, it’s almost im-
possible to fi nd the elements for many vintage bypass oil fi lters,
especially aftermarket types.
Some major manufacturers, such as AC and Fram, still make
elements for some of their early fi lter cases. Many of these are
“one-size-fi ts-all.” Often, they don’t come with the correct gas-
kets and seals to fi t every vehicle’s fi lter case.
Many people believe they’re taking good care of their vintage
car, truck or HMV by regularly changing the engine oil and fi lter
elements. If they’re using an element that doesn’t seal inside the
fi lter case, it doesn’t actually fi lter the oil!
Most bypass oil fi lters, such as those used on many common
HMVs — jeeps, WCs, early deuces, and M37s — function at all
pressures whenever the engine is running. In other words, they
are an alternate route for a portion of the oil to take as it circu-
lates through the engine. Oil is pumped into them — usually
from an oil gallery in the engine block — and then is dumped
back into the pan or timing cover after being fi ltered.
So, even if your WC is only running 5 psi of oil pressure, at
least its bypass fi lter is fi ltering. That is, assuming it has the right
element, and isn’t clogged with crud!
As you may have guessed, there has to be some sort of re-
strictor in these systems or most the oil would take the path of
least resistance, fl owing through the fi lter and back into the pan
instead of to the engine bearings.
On some fi lters, the restrictor is in the case’s center tube and
is simply a small metering hole. On other engines, the restrictor
may be a separate fi tting. It might be located either on the fi lter
case or where the fi lter outlet hose or tube connects to the engine.
On a some vehicles, the restrictor is inside the engine, often in
the timing cover.
The restrictor is very important when it comes to maintain-
ing an engine’s correct oil pressure. As many HMV owners have
found, this restrictor is sometimes missing on basket-case HMV
engines — especially jeep engines.
Additionally, there is a usually a spring-loaded valve on or
near the oil pumps in the engines of most common HMVs, and
its function is to prevent too much oil pressure in the system.
This valve is calibrated to let oil bypass back into the crankcase
if the pressure becomes too great. More is not always better, be-