Venting a Gas or Oil Tank Cap

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Article By: Tyler Malinky

If you’re building a custom oil tank, you can forgo the vent fitting and vent the cap to keep a super clean design. Also, if you need to vent a cap for a gas tank, this method also works great. What you need are a 1/8” NPT tap (which needs a 5/16 drill bit), a breather fitting (available at Lowbrow or at a good hardware store either locally or online), and a small drill bit approximately 1/16”. The breather fitting has a porous bronze screen that lets your fluid vessel breathe and equalizes pressure while helping keep the fluid where it belongs. This only takes 10 minutes to do and you end up with a vented cap that is resistant to having oil or gas splash out of it while riding.

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To start, center punch the top center of your cap so your drill bit doesn’t wander, and drill straight through with the 1/16” drill bit. This works best in a drill press. I did it in our vise with a hand drill to show how to do it if you don’t have one.

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Next, flip the cap over and use a 5/16 bit to drill deep enough for the fitting, and allow a bit of extra room so you can tap deep enough, but don’t drill all the way through your cap!

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A little cutting fluid and your 1/8” NPT tap should make quick work of the aluminum (or brass or other material).

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A blast of compressed air or a squirt of brake cleaner to get the metal chips and oil out, and you are ready to install the breather fitting. Viola, a vented cap!

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An alternate way to do it without anything but a drill and a couple of bits is to drill a hole approximately 1/8” diameter from the bottom side of the cap, but not all the way through. Stop so it is about even with the center of the edge of the cap. Then drill a small diameter hole, approximately 1/16”, through the edge of the cap perpendicular to the first hole so they just meet up. This allows your vessel to vent, and the 90 degree turn helps keep oil or gas from making its way out.

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