Servicing Your HMV Oil Filter

Back in the days of acetylene headlights, most vehicles were not equipped with filters to protect their engines from dusty air or dirty oil. This was mostly because it was simply not known how much damage dust and grit could do inside an engine such as scoring cylinder walls, scuffing piston rings, and scouring bearings and other moving parts. Of course, the tolerances in the engines of this period were loose and sloppy. They were low-compression, low-RPM, and not very long-lived.

All engines could have been fitted with air cleaners if their manufacturers had realized the need. No matter how old or primitive an internal-combustion engine might be, the air needed for combustion of fuel is sucked into its intake manifold by vacuum created when the pistons go down on their intake strokes. Therefore, an engine’s air may be cleaned of dust and grit by pulling it through a fine screen or filter before it goes down the carburetor throat.

Oil filters, on the other hand, would still have had to wait for a couple of decades.

Why? Because most early engines did not have oil pumps. Instead, they were lubricated by a splash system. There was nothing except the action of the crankshaft and connecting rods to splash oil onto pistons and cylinder walls. The crankshaft bearings were lubricated by dipping into the oil pan with every rotation. It’s difficult to filter oil that’s simply being splashed around.

With a splash lubrication system, about the only way to filter the oil is to put a screen in the filler pipe so the oil gets filtered when you first pour it in. Indeed, many engines had such a screen... which may attest to the quality of motor oil during those times. Even up into the 1970s, many gas stations still sold “bulk oil” of a nasty gray-green color from glass quart jars with open funnel-like tops, and these open jars sat outside in racks no matter what the weather.

Still, if oil is of good quality. as well as changed often (Ford specified 500 miles for the Model A), a splash system works well enough on low-RPM engines with loose tolerances and thick babbit bearings.

Even into the mid-1950s, the Chevrolet 216 engine had only a small oil pump which provided lubrication to the overhead valve rocker-arm assembly as well as to tubes in the oil pan which were aimed to shoot oil at dipper spoons on the crankshaft bearings, while everything else was still lubricated by splash. However, most other car and truck engines by this time had full pressure lubrication systems. Nevertheless, many of these engines still didn’t have oil filters or filters were offered as options or aftermarket items. There must be a way to push or pull the oil through some sort of screen or filter element. Therefore, an oil pump is needed.

As with air cleaners (another article), there are many kinds of oil filters which use many kinds of filtering media. These range from simple wire mesh screens which keep out rocks and small animals, to sophisticated canisters with replaceable paper or cotton elements that catch particles of metal and grit too small to be seen by the eye. However, there are actually only two basic types of oil filters: the BYPASS and the FULL-FLOW.

The most common bypass oil filters historic military vehicle (HMV) hobbyists will find on their vehicles are the Military Standard Junior and the Military Standard Senior. Later model HMVs, such as the M151 MUTT, the Kaiser M715, and various CUCVs such as the M880, and the HMMWV, have full-flow, “spin-on” filters... a type with which many people are familiar from changing the oil on their cars.

It’s easy to understand how both types work, because their names describe their function. The BYPASS type does exactly what its name implies: A portion of the engine oil bypasses the main lubrication system and is routed through the filter, where particles of metal and dirt are trapped in the filter element.

On the other hand, a FULL-FLOW type filters all of the engine’s oil, usually right after it leaves the oil pump under pressure. On a few engines, though, the filter is in the suction side of the system before the oil reaches the pump.

There are advantages and disadvantages to both types, so 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-flow type filters 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 filtered before it gets pumped to the bearings.

The problem is that, since all the oil must pass through a full-flow filter each time it goes around, the filter element cannot be restrictive. It cannot slow down the flow of oil.

Therefore, the filtering media of a full-flow filter cannot be made as fine as a bypass type. It can’t filter out very small particles of dirt and grit.

In addition, most engines with full-flow filters have a by-pass valve, either as part of the filter body or incorporated into the spin-on element. This valve is calibrated to open if the filter becomes clogged. This allows oil to bypass the filter media and circulate through the engine as if there was no filter at all.

A bypass filter cleans only a portion of the oil each time it makes a circuit through the engine. Therefore, it can be made finer to trap smaller particles of dirt, grit, and metal.

From late 1930s up into the mid-1960s, there was a proliferation of aftermarket bypass oil filters. Just about every company that made filter elements also offered cases to put them in. The result that was a bewildering array of filter choices. Unfortunately, for the owners of vintage vehicles today, it’s almost impossible to find the elements for many vintage bypass oil filters, especially aftermarket types.

Some major manufacturers, such as AC and Fram, still make elements for some of their early filter cases. Many of these are “one-size-fits-all.” Often, they don’t come with the correct gaskets and seals to fit every vehicle’s filter case.

Many people believe they’re taking good care of their vintage car, truck or HMV by regularly changing the engine oil and filter elements. If they’re using an element that doesn’t seal inside the filter case, it doesn’t actually filter the oil!

Most bypass oil filters, 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 circulates 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 filtered.

So, even if your WC is only running 5 psi of oil pressure, at least its bypass filter is filtering. 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 restrictor in these systems or most the oil would take the path of least resistance, flowing through the filter and back into the pan instead of to the engine bearings.

On some filters, 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 fitting. It might be located either on the filter case or where the filter 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 maintaining 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, because too much oil pressure can damage front and rear crankshaft seals, letting oil leak onto the clutch, and leave your engine low on oil out on the road.

Occasionally, this valve may become jammed, especially on vehicles that haven’t received proper maintenance and regular oil changes. I once bought an M37 that always pinned its 60 psi oil pressure gauge, even at idle. I thought this was a typical malfunction of M-series gauges and checked it with a mechanical gauge. It pinned at 120! This explained the truck’s insatiable oil consumption — and all of the puddles it left when parked.

It’s not a good idea to try to increase your vehicle’s oil pressure by stretching the spring in this valve or by adding washers to it. (This, is a common trick used by some morally-bankrupt vehicle sellers to boost oil pressure in a worn out engine.

When should your oil filter be serviced? Every time you change your oil — hopefully, that’s before it begins to look like tar! Keep in mind that motor oil can look clean but still be worn out. This is why you should establish a regular changing schedule.

For HMVs that are driven regularly, 3,000 miles is a good interval to change the oil and service the filter. For vehicles only used for short trips and that never get fully warmed up, moisture will contaminate the oil. This will form acids that eat away at bearings and cylinder walls. For vehicles used for short trips, you should change the oil more often.

An old shade-tree practice was to service the oil filter only every other time the oil was changed. This is not a good idea. If you don’t service the filter whenever you change the oil, you’re usually leaving at least a quart of dirty old oil in the engine.

If you have just acquired an HMV with a bypass oil filter and/or have had no previous experience with this type, here is the service procedure:

That’s all there is to servicing most HMV bypass type oil filters.

While many people have serviced this type on their civilian cars and trucks, others have always had their oil changed at a garage or quick-lube joint. Changing the oil and replacing the filter on their M715, M151, M880, or HMMWV may be a new experience for these people.

*First, you need a good-quality oil filter wrench. There are several types, but a steel band model usually works better than a nylon strap type or those that look like big sockets. Don’t buy a cheap wrench. It will bend or break under stress.

Hopefully, this covers just about everything you might have wanted to know about oil filter service on your HMV. 

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