View Full Version : Oil Bath Air Cleaners 101

08-25-2018, 10:07 AM
It is funny how a lot of us end up talking about the same subject at the same time. The discussion of this week has been oil bath air cleaners. Oil bath air cleaners, Donaldson oil bath air cleaners in particular, are an item that look simple, but have a lot of sophistication once we start looking at them.

Donaldson air cleaners rely on the fact that dirt has weight and anything with weight has inertia. We dont think of dust and grit as having inertia, but particles of crud tend to keep moving in the same direction just like everything else. Donaldson air cleaners take advantage of this to make dirty, gritty, dusty air into something that our Jeep carburetors will use and not grind up the innards of our engines. In fact, a guy named Dyson is making a lot of money selling vacuum cleaners that use this same principle.

The oil bath air cleaner consists of a canister having two concentric cylinders inside it. Dirty air enters the cleaner through a number of holes in the side of the outer canister. In many of the civilian Jeep applications, these holes are up against the wheel well so that as little rain or other water as possible will get into the air stream. On the M38 and M151 military Jeeps, which go swimming on a regular basis, there is a water separator to make water entry less probable.

Air enters the outer canister, it swirls around the inner metal cylinder and is drawn downward toward the bottom of the air cleaner. This cyclone action uses the inertia of the larger dust and grit particles to throw them against the wall of the outer cylinder and then lets them drop into the oil puddle in the bowl at the bottom of the filter. At the bottom of the outer cylinder, the air stream makes a 180 degree turn, heading upward into the inner chamber of the air cleaner. Once again, inertia comes into play. Most remaining dust and grit does not make the 180-degree turn and drops into the oil puddle.

Now, the mostly clean air enters the inner canister where it goes through the steel wool, or whatever material, to the air cleaner outlet at the hose to the carburetor. This material acts again as a maze for remaining dust particles and provides acceptably clean air to the engine.

Yep, its messy. But it has the advantage that it is a fail-safe design. Even if the oil bath at the bottom gets so full of dirt, dust and grit that it turns into a solid block of grease, the inertia action of the filter and the steel wool maze will still remove a bunch of dirt from the air stream. A paper filter is a fail unsafe device if it gets dirty the air stream is blocked and the engine quits. This is no big deal if you can run down to the parts store for a new Fram air filter, but if you are in West Waziristan BFE .. .

And, once again, the CJ3 people have done a much better job of describing this that I can. Here is a link to find out everything you would ever want to know:


08-25-2018, 07:16 PM
Thank you again for your leg work on this subject. The subject is very interesting, and it is incredible that 75 plus year technology is that efficient. I see engines everyday that have been deployed to the great sandbox of Iraq or Afghanistan. Upon servicing the engine, they pull the intake tubes off to access the valve cover. There is always a fine coating of dust inside the tubes. This dust ingestion does cut down on the longevity of the Caterpillar engine. It is unusual to see an engine with more than 5,000 hours without going in for overhaul. It would be very interesting to do an experiment on setting up an engine with an oil bath cleaner.

08-26-2018, 01:53 PM
We kind of poke fun at the "steel wool" in the inner filter - "It keeps out birds..". but by the time air gets to it, the birds have been filtered out and have gone swimming in the oil puddle. The oily steel wool maze is a highly serpentine air path and dust particles have to hit a lot of oily steel wool surfaces to get to the outlet.

08-26-2018, 02:00 PM
My Internet was acting a fool yesterday, probably from a bunch of lightning storms in the early morning. I could not upload these two pictures when I posted the first thread.

A. You can see the air inlet holes on the back side of the air cleaner with the hood that acts as a cover to help keep water out of the inlets

B. In this picture you can see the two concentric cylinder construction. Inlet air comes down through the outer cylinder, then immediately turns the corner and goes back into the steel wool/mesh filter material.

05-15-2019, 11:34 AM
If you had to choose between this air cleaner and a factory K&N setup which way would you go?

05-15-2019, 03:06 PM
Awhhh man - do I have to make a choice?

The Donaldson/Houde centrifugal oil bath filter was original on the truck and I've modified the air cleaner in the truck to look like it might have been the original - but real Jeep connoisseurs will see the L/F head adaptation. But - never say "Willys didn't...".

Anyway - opinions are like armpits, I have couple and sometimes they stink . Here's one.

If you're going out and play in the sand, dust and dirt big time, there are a lot of folks who have evidence that the oil bath cleaners do a better job in getting the grit and crud out of air - especially when they have loaded up a bit. (see GmWillys comment about Caterpillar diesels from Iraq/Afghanistan and 5000 hour overhauls!)

If you're just playing in the dirt a little, I doubt if you would see any reason to change from the factory filter - just keep it clean (keep the oil bath clean too!). Just puttering around town - leave it as-is.