Introduction: Make an Oil Bath Air Filter for Your Motorcycle or Generator From a Coffee Can.

I've always liked the look of a stripped-down, barebones motorcycle, but I've never been fond of 'pod' style air filters. When I modified my bike, the big air cleaner box was the first to go. Running anything that sucks air into a tight tolerance environment is, frankly, a bad idea. So I've been considering air filtration options for a while that might fit the 'look' of my bike and nothing seemed to jump out at me.

Then, I had dinner with a friend of mine who just bought a 1955 Chevrolet in complete, stock condition. He lifted the hood and, behold, there was an oil bath air filer perched atop the engine.

For those that are too young to have even heard of an oil bath air filter, they were the dominant means of removing dust particles from the air being sucked into an engine through the carburetor (another antiquated device....). Included is a quick drawing of a rudimentary oil bath filter that uses a canning jar.

The premise goes like this:

There's a reservoir can, steel wool, intake pipe and a small amount of oil. The reservoir can has about an inch of oil in the bottom of it and a layer of rough, steel wool about half-way up from the bottom. In the center of the reservoir is a pipe. One end of the pipe is suspended just above the oil in the bottom of the reservoir can. The other end of the pipe connects to the carburetor.

As the engine turns and creates a low pressure in the combustion chamber, air rushes into the engine through the carburetor. In our Oil Bath Filter scenario the air rushes into the top of the reservoir can, through the steel wool and then is forced to take a sharp 180 degree turn to go up into the intake pipe that feeds the carburetor.

As the air turns the sharp corner to enter the intake, particles in the air drop out and stick in the oil at the bottom of the reservoir.

These air filtration systems were used in VW products until pretty recently. In many countries across the globe the Oil Bath Air Filter is the preferred means of filtration. There's a long and storied debate over the effectiveness of oil bath over paper filters. In very dusty environments a paper filter is prone to clogging in minutes whereas an oil bath filter can operate for hours before the oil needs to be changed. I AM NOT interested in opening the debate over which filtration is better. I will say this though, I've seen engines with oil bath filtration go well over 300,000 miles without any ill effects. I've also seen the same with paper filtered engines.

I've chosen an oil bath for aesthetics, not because I think it's superior to paper filtration. With that, let's get going.

What you'll need to make an Oil Bath Air Filter for a single-cylinder motorcycle (or similar engine):
Pipe that fits over the carburetor
90 Degree elbow
Coffee can
1/8" Hardware cloth
Scotch-brite or very rough steel wool
Hose clamps
Buckskin (optional)

Tools you'll need:
Tin Snips (to cut hardware cloth)
Torch, Flux and Solder (optional)

Step 1: Create the Intake Tube

The pipe that connects to the carburetor is what I call the intake tube. I used some left over copper from a friend's boiler project as my intake tube. In my case the outside diameter of the carburetor opening was just a hare smaller than 1 1/2", so copper pipe with a 1 1/2" Internal Diameter worked really well.

I created a tube that would extend into the coffee can. Be sure to cut the pipe so that it doesn't reach all the way to the bottom of the can. There's needs to be about 1 to 2 inches between the bottom of the intake tube and the bottom of the can.

To get it to attach to the carburetor, I created about 15 'kerf' cuts on the carburetor end of the copper pipe and used a hose clamp to tighten it onto the mouth of the carburetor. For a little extra measure I wrapped the carburetor opening with some buckskin I had on had to act as a gasket between the carburetor mouth and the copper pipe.

Step 2: Create the Primary Filtration

In addition to having a cap of some kind, oil bath filters have a primary filtration to catch the 'big chunks'. In many instances it's a layer of heavy steel wool (similar to a 'Brillo' pad or coils of metal filings you find near a lathe).

My oil bath filter is open on the top which means just about anything can get sucked in. Personally I love the growling sound of induction that is produced when rolling on the throttle of my motorcycle. The open top of the filter allows more air flow into the filter and provides some great resonance for that rewarding growl when you really give it the berries.

To create the primary filter I simply cut 2 circles of hardware cloth that tightly match the inner diameter of the coffee can. Between the layers of hardware cloth I use some 'Scotch-brite' to catch the finer chunks of road dirt.

To keep the primary filters out of the oil bath at the bottom of the can I simply put a hose clamp on the underside of the lower piece of hardware cloth. That should hold it!

Step 3: Hook It Up, Fill It and Go!

The best thing about creating an oil bath filter is how quick and easy it is. You don't have to worry about getting a super tight seal on a paper filter. Now that the primary filters are in place it's a short step to completion.

Slip the coffee can under the intake tube and push it over the hardware cloth (primary filter). If you have a tight enough fit you won't need to attach the can to anything to hold it in place. If not, you can easily use some wire to hang the can on the intake tube. In my case, it's held in place by the primary filter at idle. When you roll on the throttle the vacuum actually holds the can in place as your driving.

When the can is in place, dump a several ounces of oil over the hardware cloth and scotch-brite sandwich (primary filter) and you're all set. Grab your helmet, kick your bike over and roll on the throttle with peace of mind.

Step 4: Important Final Notes

When your engine runs without an air cleaner the air/fuel mixture is really bad. Your engine runs 'lean' and thus it will run hot, backfire and burn up spark plugs. The factory uses the correct carburetor setting based on the amount of vacuum that's created from having an air filter in line before the carburetor.

When you add your new oil bath filter you need to make sure you've got proper air flow. If the can is too small there's not enough air flow and your engine will suck the oil up from the bottom of the can. In addition you'll run 'rich' (too much fuel, not enough air) and foul your spark plug. Too much air flow and your engine will run lean.

My scenario worked out perfectly. My coffee can has a 3" opening which is twice the size of my intake tube. The bike runs perfectly with this setup.

This type of setup is great for generators and stationary engines. Think of all the air filters you *won't* be using this year at Burning Man!