Proper Fuel Cap


Dec 27, 2019
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Blue Springs, Mo.
Question...which gas cap should be used on 2 vent fuel tank. My '65 300L has the original vented cap. Recently brought back to life a gorgeous ' 67 Plymouth Sport Fury convertible that had been sitting in a garage since 1981. Naturally had to replace the fuel tank, lines, fuel pump etc. I've noticed it has an aftermarket fuel cap... probably from the '70's that is Not Vented. This car has the same 2 vent gas tank as the '65 300. I know they're different tanks but have the same 2 vent lines coming out of the tank. Thought maybe since the fuel filler tube sets so low...coming out behind the license plate...they may have used a non vented cap?? Thought I'd run this by the C-Body guys out there. Sure don't want the new tank to implode!!
When carbon canisters came to be used, the fuel tank caps changed too. No way to put them on an earlier tank.

I'm thinking the cap should be "vented", but also has a ball check valve to prevent fuel slosh from getting out? If the tank has the two vent lines, that should keep the "vents" open, no matter what, I would suspect?

Take care,
This non vented aftermarket cap is kinda hard to take off and put on. Thanks CBODY67. I'm thinking I should put a vented cap on it.
Just checked listings in RockAuto. Seems the '66 cap Gates lists is non-vented, but the '65 cap is "vented, anti-surge" (which is what I remember being on our '66 Newport) (which has the check valve I mentioned).

The buyers guide listing shows the "vented anti-surge" cap going from 1957-1970, but the non-vented cap shows 1966-1970. So, some overlap there.

Is the installation difficulty due to a fiber sealing gasket on the cap? If so, it might compress with time, possibly.

Take care,
Stant G-32 non-vented works well on my 65 Dodge, w/ similar tank/filler placement . . . I think I finally came to the conclusion that non-vented was used on the "regular" tanks, but vented was used on station wagons. But, I could be wrong . . .
Thanks for the info on this. The non vented cap on the car now is aftermarket from the '70's. The fitment problem getting it on and off is something to do with the tangs engagement with filler tube. Not the gasket on the cap. Thinking about getting a vented anti surge cap CBODY67 mentioned on a 66 Newport. Thanks again guys for your info.