Restored German Oil Bath Air Cleaners

Not a week goes by without someone inquiring about the restored German oil bath air cleaners that are offered for sale here at There’s a common theme among these questions. People seem to be curious about how restored German oil bath air cleaners work in terms of their design, and they want to know why factory original is better than an aftermarket unit.

I was doing a bit of research to help best answer these questions and I ran across Bob Hover. He has a fantastic article that really breaks down a lot of good information worth reading.

“You have been misinformed as to the principle of the oil-bath air-cleaner. There is no vaporization and condensation of the oil bath. I suppose horsehair may have been used for an air filter by someone but it was not Volkswagen.

The oil-bath air-cleaner is a classic example of a two-stage’ kinetic filtering element, removing virtually all particulate contaminants from the air at all engine speeds.

Incoming air is forced to follow a vertical descending path toward the pool of oil then drawn upwards. Having a mass several million times that of a molecule of air, the inertia of the dust particles makes it impossible for them to follow the abrupt change of direction in the air-stream, causing the particles to strike the pool of oil where they become trapped. This works best at high rates of air-flow.

The coir filter element, which forms what is termed a ‘labyrinth filter’, applies the same principle but in a different manner. The coir element forces the air to change direction many times. The fibers are coated with kerosene. Dust particles collide with the fibers and are trapped by the kerosene.

The spec for cleaning the coir filter is to immerse it in kerosene, allowing it to soak for up to half an hour. It is then sloshed repeatedly and allowed to drain. This was done twice a year under normal driving conditions, as often as deemed necessary under dusty conditions.

In use, particles of dust trapped in the oil bath cause the level of the oil to rise. Under severe conditions it may require cleaning on a daily basis. (Moisture does the same thing. In a rainy climate it isn’t unusual for the air cleaner to accumulate a quart of water per month.)

Under Volkswagen’s original apprenticeship training program the effectiveness of the oil-bath air-cleaner was demonstrated by removing the sludge from a the oil-bath and coir filter, flushing it with solvent and examining the residue. A low-power microscope was needed to observe the smallest particles.

The same principle is used to clean the air for large stationary engines and for air conditioning applications, in which a recirculating water-bath may be used instead of oil, and the air may be forced past as many as two dozen up-down baffles, removing even microscopic particles of low density such as pollen. In some systems the water-bath is sealed with a thin film of mineral oil. Trapped particles fall thru the oil and are removed by the recirculation of the water beneath the oil film. I understand special silicone-based oils are used in modern HVAC systems but non-human applications such as large stationary engines continue to use mineral oil. Residential HVAC systems typically use labyrinth-type filters, designed primarily to catch fibers rather than particles.

Paper and foam filtering elements are based on the labyrinth principle. The effectiveness of the oil-bath air-cleaner is superior to that of the typical paper or foam filtering element. Paper air-filtering elements came into use when they became effective at trapping particles of a certain size. Oil-bath filters will trap smaller particles but there is no evidence of accelerated engine wear for particles below a certain critical size.

Air filters for rough service (armored vehicles, farm machinery, etc) where an oil-bath would be unsuitable, and high-volume applications (turbines, etc) use the same physical principle of mass-differentiation, typically drawing the air through several stages of centrifuging during which the greater mass of the dust particles causes them to be separated from the air-stream. Although such air-cleaners may be powered or static, they are often called ‘turbo’ air-cleaners. They are often used in conjunction with disposable labyrinth-type filters. For Volkswagen owners running off-pavement, the static type of ‘turbo’ air-cleaner used on Ford tractors has proven most effective…”

(Read the full article)
Thanks Bob for writing such great info. Rest in peace my fellow VW pal. Please keep those emails coming if you have more questions.

Posted by Eric Shoemaker

Hello, I'm Eric. I started Air-Cooled Artifacts (previously, and Lane Russell). I drive a '67 Beetle daily and love to share vintage Volkswagen stories with the world.

  1. Great article. I’ll take my German oil bath anyway over aftermarket junk.

  2. Yes , I would like to buy 2air cleaner stickers at 8.00a piece , whoknew1085@ thank you

    1. James,
      I just sent you an email.

  3. how do you clean a oil bath air fliter????

    1. Alex,
      Are you speaking about the internal structure? If so, my process is to take the unit down to bare metal; followed by a soak in a solvent. This will loosen up most of the gunk in the horse hair.

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