VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Vented Fuel Cap

I ordered and received a new vented fuel cap from Chev of the 40's for my '37 Master Coupe (GB).

I needed to replace the non-vented aftermarket cap because it would allow fuel to splash out of the filler when the tank was fairly full and I made a left turn. I knew that if I added a gasket to seal that cap I would shut off the vent function and starve the fuel flow to the carburetor.

Now I am not sure if I have the right cap or perhaps there is a problem with the filler neck. The new cap is Chev of 40's part # 595857 which is the same part number that is listed in my 1940 parts catalog. Unfortunately there is also the possibility that the fuel cap was not made properly as I am experiencing with various reproduction parts.

The gas cap does not fit snugly on the filler neck. It seems very lose and feels like to could actually rotate off. So I added another layer of gasket material to it and it feels tighter. I am able to drive the car without any signs of fuel starvation so I think the vent is working properly. I have yet to fill the fuel tank completely to confirm if it will prevent the fuel splash-out.

Yesterday after I had driven the car for about a half hour I decided to check the fuel cap to see how it was fitting. When I released it there seemed to be pressure in the tank.

1) What can I do to make the cap fit tighter? Does anyone sell the rubber seal as a separate part?

2) Is the pressure build up normal? This means that the vent function is really a check valve and only allows air to enter the tank.

The reason for a vented cap is/was to allow air to enter to take place of the gas consumed. Also if the gas is heated and expands it allows the air to escape.A passage of air should not be heard when the cap is removed as the vent is always in play.
That being said it depends on the method used for venting and adding gasket material may be blocking the vent.

Thanks Gene,

It appears that this is a case of "one part does not fit all" from the aftermarket.

I removed the extra gasket I added so I would prevent the pressure build-up. The fuel sloshed out even with only 1/4 tank of fuel.

I then tried the new cap on my friend's 1954 3100 pickup. It was a slightly tighter fit than on my car but no where near as tight as his cap on his truck. When I tried his gas cap on my car it seemed to seal fairly well.

So it seems that I need to start looking for a NOS cap. I don't even want to start the debate with Chev of the 40's about whether the cap they sent is correct. They have 2 standard answers in these cases. 1) That is what our application data shows. 2) We will have to check with our supplier.

The caps I see on eBay and the Filling Station look like the cap that Chev of 40's supplied. So it does not make sense to order one of them.

Once again, thanks for the help. And it was great to see you in Des Moines for the Middle West Meet.

Rusty,

It seems strange to me that at a 1/4 of a tank of gasoline it would slosh out on a left hand turn. Mercy! Does your tank have baffles?

Must be the ethanol.

Charlie

Rusty you might try NAPA or other part stores.

Hi Denny,

Thanks for the suggestion, especially NAPA.

Rusty

Just want to close the loop on this topic.

Sunday I got the proper gas cap from AutoZone. The key is that I needed what they call a "shallow cam" vented cap. They were able to do a cross reference between their part number and a Stant #10623 cap.

The cap that I got from Chev of the 40's was a "regular" cam. The dimension between the gasket and the metal tabs on the cap was larger so it would not lock on the fuel filler neck.

The vent is a one way check valve so it will only let air into the tank. There is a slight pressure build up in the tank due as air temperature increases.

Yesterday I drove the car about 100 miles with temperatures in the upper 80's with no problems. We had an informal lunch gathering of Cedar Valley VCCA members. Great day!

Thanks for letting us know how things worked out for you.

Could you post the AutoZone p/n for the cap. That would save us with going through the "Does it have A/C" routine.

Good suggestions! Thanks.

It is a Duralast 6023 fuel cap. I paid $6.39 for it.

They do not have listing by application for my car.