MILITARY VEHICLE AIR CLEANERS

Most common vehicles in the historic military vehicle (HMV) hobby have at least three types of filters to protect their engines and other mechanical components: oil filters, air filters, and fuel filters. In Military Vehicles Magazine issue no. 205, we learned how to service and maintain oil filters (“Bypass or Full Flow: Servicing Your HMV Oil Filter,” pp. 22-27, October 2019). This article will deal with air filters.

Some vehicle service and maintenance tasks are so mundane, boring, or dirty that we often neglect them. Examples are lube jobs, checking and/or changing the gear oil in transmissions, transfer cases and axles, and draining, flushing, and renewing engine coolant, as well as brake fluid. Ironically, while these chores can be dirty, and boring, relatively simple maintenance do the most for keeping our HMVs on the road. These chores guarantee our vehicles’ good mechanical condition and that they will have long and dependable lives. Regardless, the item at the top of the list of neglected chores is servicing a vehicle’s air cleaner, whether oil-bath or dry-type.

Back in the early days of Military Vehicles Magazine, I hosted a question and answer forum for mechanical matters. One of the points I tried to make — and still believe today —– is that there aren’t many stupid questions except the ones that don’t get asked.

One of the letters I received back then was about an oil-bath air cleaner on a CJ3A (which, as most HMV hobbyists know, is a fraternal twin of the M38). The question was, “Where does the extra oil come from to raise the level in the oil cup from ‘Safe’ (or ‘Normal’) to ‘Danger’ (or ‘Service’)?”

The answer is that the oil level doesn’t rise because extra oil mysteriously appears — or as the reader asked, “is possibly sucked up from the oil pan” — but rather, because of dirt and dust particles trapped by the air cleaner doing its job and settling to the bottom of the oil cup.

This question was of interest to me because my first jeep was an M38. As its young and innocent owner, I recalled reading the words embossed on the air cleaner’s oil reservoir: SAFE LEVEL and DANGER LEVEL. I had also wondered why the danger level was higher than the safe level. In other words, where did that “extra oil” come from?

Answering that question and, hopefully, many others about air cleaners or air filters, is what this article is about. In past articles I’ve mentioned various vehicular components that seem shrouded in mystery in regard to how they work or what their useful purpose might be. I’ve also written about components that seem so long-lived and trouble-free that their care and service is often overlooked or ignored until they finally wear out and fail… wheel bearings and universal-joints being good examples.

THE LEAST-SERVICED COMPONENT

But, if you asked me what I thought