How to make a Vented gas cap

[cyclespun]

What’s up guys,

I recently got a gas cap as a gift and want to run it on my tank. It doesn’t look to be vented and I am curious what’s the best way to make it vented and how to go about it as far as drilling a hole into it. I hear a lot of guys run into leaks coming out of the top of the cap. But I also see some vented gas caps with just one hole drilled into the underside of the top of the cap without a second hole. I have yet to see a good vented gas cap thread of a “how to” with photos, so thought I’d share photos of the process afterwards as well in this thread. Thank you in advance.

[1954bmw]

Do you see the gap in the gasket, could that serve as a vent, looks like it would leak air??

[confab]

Do you see the gap in the gasket, could that serve as a vent, looks like it would leak air??

^ Agree.

OR:

If it doesn't bleed air now.

Silicone a cork gasket in there (so it doesn't slide) with a gap in it to permit it to vent?

Since it has an emblem for positive orientation, align the slot in the new gasket with the top of the neck.

[cyclespun]

Okay I think I get what you guys are saying. The gasket that it came with was just a rubber flat washer style one... I thought it would need some sort of hole drilled...

[confab]

Okay I think I get what you guys are saying. The gasket that it came with was just a rubber flat washer style one... I thought it would need some sort of hole drilled...

You can do that on caps that are of two piece construction, with a pathway from the inside to atmosphere without drilling a hole in your chrome top.

If it is two piece, you would normally just connect that path by drilling a small hole.

Yours looks like a single piece, which would have you drilling a hole straight though it.

[cyclespun]

Yeah so mine is one solid piece. So just puttin a gasket in it wouldn’t do it? Like below. Laugh at my stupidity but I’m at least tryin haha. I’ll drill a hole if I have to I just have heard it can squirt out all over fresh paint which would suck

[cyclespun]

[confab]

Yeah, that will work. Now you need to make the vent in the gasket.

Silicone that in there and put a slot in it for the tank to vent and you're fine.

If that hole will connect the inside of the tank with atmosphere? You're good.

Otherwise, make it so.

[1954bmw]

Don't know about that silicone, I don't think silicone plays nice with gas fumes???

[cyclespun]

Do you guys think this will work. I drilled a 1/16 hole through the top to the inside of the cap. Which sits underneath the rubber gasket. There’s basically an air passage or moat that lies underneath the gasket about 1/8 deep. I then slotted another air passage, basically a V shape underneath where the gasket lies so if pressure builds it will hopefully push under the gasket and out of the hole...

[cyclespun]

[cyclespun]

Idk if you can tell from the photos but that rubber gasket is forced or can only sit above that moat which is allowing a air/gap of space underneath it. So I channeled an V entrance to that and drilled the hole exiting that moat for air to escape

[confab]

Do you guys think this will work.

Good idea. It will work fine as long as the tank is vented to atmosphere.

It is't not, it will vac lock on you, or might even build enough pressure to force fuel past your needle seat on a hot day when it is parked.

You can turn it over and blow through the petcock to test it. As long as it vents every time you put the cap on, you're golden. If it doesn't? It's gonna blow you crap.

[qawsed]

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.


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.
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.