Is this gas cap vented
A brake tube is indeed flared, but it needs the proper fitting to seat into. A nut isn't it
An inverted flare (brake) fitting on a tube MUST have the female "mate" as shown here
If you just screw the tube nut into a standard nut, there is no internal seat against which it can seal. Pipe thread needs no such seat because they are tapered, and go tighter as you screw it (pipe fitting) in.
So if you can either create a pipe thread, as I said earlier, say, weld a nut or other "boss" to the filler, then chase it for 1/8 pipe, and use one of the brass adapters .....or...if you can braze the brass adapter directly to the filler.
To me, it would be far easier just to do it exactly as the factory did.......braze a short steel tube to the filler "around back" out of sight, then use a very short rubber hose coupler to hook up the remaining vent tube. Very important to bring the tube up "upside down U" then back down through the gasket "outdoors." The originals up through 69--part of 70 went through the floor gasket and ended "open end" in the inside of the rear frame rail stub. I assume this was for protection. You don't expect "much" fuel to ever come out, there.
For these kinds of jobs I use what most used to call "silver solder" meaning hi temp silver braze. I think you could use MAPP gas, but oxy--acet is the usual heat. I don't believe LP (propane) would do it.
A brake tube is indeed flared, but it needs the proper fitting to seat into. A nut isn't it
An inverted flare (brake) fitting on a tube MUST have the female "mate" as shown here
If you just screw the tube nut into a standard nut, there is no internal seat against which it can seal. Pipe thread needs no such seat because they are tapered, and go tighter as you screw it (pipe fitting) in.
So if you can either create a pipe thread, as I said earlier, say, weld a nut or other "boss" to the filler, then chase it for 1/8 pipe, and use one of the brass adapters .....or...if you can braze the brass adapter directly to the filler.
To me, it would be far easier just to do it exactly as the factory did.......braze a short steel tube to the filler "around back" out of sight, then use a very short rubber hose coupler to hook up the remaining vent tube. Very important to bring the tube up "upside down U" then back down through the gasket "outdoors." The originals up through 69--part of 70 went through the floor gasket and ended "open end" in the inside of the rear frame rail stub. I assume this was for protection. You don't expect "much" fuel to ever come out, there.
For these kinds of jobs I use what most used to call "silver solder" meaning hi temp silver braze. I think you could use MAPP gas, but oxy--acet is the usual heat. I don't believe LP (propane) would do it.