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Pots and Cans

Step-by-step fabrication of swirl pots and catch cans/oil-air separators...

By Michael Knowling

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This article was first published in 2002.
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Catch cans and swirl pots are relatively simple devices that many people choose to piece together in the backyard. For a positively slick job, though, you can't beat custom fabrication by a professional workshop. To highlight this, we went along to Adelaide's ASE (Auto Sport Engineering) to see how cans and pots are pieced together...

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ASE offers two different products to prevent blow-by oil entering your engine's air intake - a catch can and an oil-air separator. What's the difference, you ask? Well, a catch can receives blow-by oil and allows fumes to vent to atmosphere after being filtered. On the other hand, an oil-air separator internally filters the blow-by oil and allows only clean air to enter the car's induction system. This is the favoured approach from an emissions and legality perspective, but it doesn't look as siik when you pop the bonnet.

So how is a quality catch can/oil-air separator made?

Catch Can/Oil-Air Separator

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First, ASE takes delivery of a custom aluminium tube that is made to their specified length. The catch cans/oil-air separators seen here are 100mm in diameter and 170mm tall, giving an internal volume of around 1.5 litres. A larger version that's 200mm tall gives approximately 2-litre capacity. Following a construction plan, fabrication kicks off by drilling a blow-by inlet hole at one end of the tube.

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Once drilled and de-dagged, a very tidy looking boss is used to cover the hole and provide means of hose attachment. This boss - which incorporates a ¼ or 3/8-inch BSP or any "dash"-size (as in a -8 line) internal thread - is then welded around the hole. ASE performs TIG welding for its fabrication of catch cans/oil-air separators and swirl pots; TIG results in a much tidier weld.

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Next, a pair of aluminium end caps (which look like larger versions of fence post caps) step into the picture to form the body's top and bottom surfaces. Both the top and bottom caps need to have a hole drilled through their centre - these, respectively, form the top air outlet and bottom oil drain hole. The cap that will be used to form the bottom of the separator then has a threaded boss welded to its outside - this, once sealed with a plug, creates an oil drain facility. Note that this bottom fitting can alternately be used to redirect oil to the sump if desired. Both the top and bottom end caps remain separate from the outer separator body - for now.

Next, attention is turned to the core of the separator...

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Inside its separators, ASE employs a central tube that extends from the base out through the top. This central tube forms the air outlet. The idea is that blow-by enters the separator body from the side and oil drains down to the bottom; meanwhile, air can escape up through the central tube via a series of holes along its length.

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The central tube - which is 25mm diameter - first has a series of holes drilled along its length. These holes are said to provide sufficient flow to prevent pressure build-up in the crankcase. Next, a 'V' is cut into the base of the tube - this allows any oil that finds its way into the central tube to drain down and mix with the rest of the contained oil.

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Once the holes are drilled and the V is cut, the central tube is TIG'd to the bottom end cap at a right angle. The base of the central tube aligns with the centre of the drain plug. The bottom cap/central tube assembly is then welded to the base of the separator body, ensuring that the holes in the central tube face opposite to the blow-by inlet. This reduces the amount of oil that finds its way inside the central tube.

Up until now the catch can and oil-air separator have followed exactly the same fabrication plan. Here's where they deviate course...

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The body of the oil-air separator is filled out using a special oil and fuel resistant foam filter. Here you can see the outline of the separator body being cut out of a foam block. A small hole is then put through the centre of the foam cutout, allowing it to be slid over the central tube. Two of these 'foam donuts' are placed one over the other inside the separator body, providing filtration all the way from near the base to the top. As mentioned, the clean air that escapes up through the central tube is then routed back to the car's induction system.

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In the case of the catch can, there is no filling necessary inside the body. Instead, a pleated K&N-type filter is clamped over the top of the air outlet, preventing contaminated fumes venting directly to atmosphere. So, in essence, one filter is substituted for another - an external pleated filter replaces the internal filtering found inside the 'plumb back' oil-air separator.

Swirl Pot

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The 2-litre external fuel swirl pot fabricated by ASE holds true to the name - the fuel return fitting in the side of the tank is angled so that the fuel is swirled around inside the tank. This helps prevent undesirable fuel aeration. The other advantage of the swirl pot is that it acts like a conventional surge tank - it helps ensure constant fuel flow to the engine even when the fuel is be sloshed around inside the main fuel tank.

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Swirl pot fabrication kicks off with the baseplate. Construction measurements are marked out on a 3mm thick sheet of aluminium, which is then cut square in a heavy-duty cutting machine. Once cut, the edges are ground smooth and a mounting hole is drilled in each corner.

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Next, the aluminium swirl pot body (which, again, arrives at ASE from an outside supplier) has four fuel fitting locations marked out - generally, the top fitting takes a feed from the 'lift' pump in the main fuel tank, the bottom fitting connects to the high-pressure pump that feeds the injectors and the two fittings entering the pot at an angle are reserved for the fuel rail return line and a return line back into the main tank. In any case, however, you can configure the fittings in whatever scheme you like. Each mark is then drilled and the edges are tidied up.

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The faithful TIG welder then unites the open end of the swirl pot body at a right angle to the baseplate - note that, in this particular product, nothing goes inside the swirl pot.

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The final step is to attach the fuel fitting bosses to the outside of the pot - this is done in the same way as in the catch can/oil-air separator. Once the greasy fingerprints from the handling process are buffed off, the job's done!

Cost and Cosmetics

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ASE retail both the catch can/oil-air separator and swirl pot for AUD$200 (including GST) each. Also thrown in on the deal is a 'T-bar' stainless steel clamp with a rubber insert - ideal for mounting either the pot or can. Compared to various other prices we've seen for custom catch cans/oil-air separators and swirl pots AUD$200 represents quite good value - and it's a much more attractive alternative to slapping together your own...

Lastly, if you really want to impress your mates, you can take the next step of cosmetic appeal - both the pots and cans lend themselves perfectly to a high polish...


ASE (Auto Sport Engineering)
+61 8 8299 0320

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