Gas cap - Vented or Not

This is an old thread, but it comes up in Google search results.

To offer clarity, you can group which tank and which gas cap to use on the following criteria-

1970 Federal and all earlier use a tank without the 1/4" fitting located above the sending unit. They require the usage of the correct filler pipe which has the vent fitting up high and the 1/4" metal line affixed to the filler pipe and leading downward through the filler tube seal where it ends in the frame rail...just an open line.

1970 California and all 1971 A-bodies use a tank with 4-corner venting. There are 4 fittings on the driver side of the tank that lead to a vapor separator inside the trunk. There is a single line leading from the separator up front to the engine bay, where a rubber line continues to the crank breather assembly. Only the OEM breather has the fittings. On these models the carb bowl vent connects to the fuel pump housing on Slants and to the same crank breather on small-blocks.

This is a crude, poorly functioning system that caused hard starting after heat soak. It also is the beginning of the end for the average backyard mechanic to understand what he is looking at.

1972 A-bodies...all of them...all year, used a tank with a 1/4" vent fitting located above the sending unit location. The 4 corner system with the lines and the vapor separator went away on A-bodies. The line from the tank vent led up front as in the previous 2 years. The charcoal canister is added and the gimmicky crank breather idea is eliminated. The canister stores carb vapors, tank vapors and is trigged to digest them once the engine is running via vacuum signal from the carb.My Valiant is a very early '72 built 8/71 and it has a charcoal canister.

1973 to '76 are just an continuation of the '72 system. There is a change in the filler neck for 1975 and the requirement to use unleaded which required a cap change as shown above.

Use the fuel cap as outlined above. There shouldn't be any need to buy several caps to figure out which one is the correct one to use if you look at the pictures above and understand what is being said here.

If you don't want to try to understand what is being talked about here, or you simply don't wish to conform, you can build your car any way you like. But, there won't be a cap that just lets the vapors OUT of the cap unless you throw the cap away and stick in a rag in it. And, if you try to use a tank with no vent provision, i.e. no vent on the tank and a filler neck without a vent, you won't like it. It'll nearly blow the cap off when you unscrew it on a hot day and the pressure might blow the needle off the carb seat and flood it.