The Stovebolt Site Forums The Shop Area General Truck Talk Fuel leak at tank

I have a fuel leak where the tube goes into the bottom of the gas tank. It is a aftermarket gas tank and I used a threaded brass nipple to go into it with a gas shut-off valve attached to it. I did use Permatex fuel resistant gasket sealer on the threads which are a pipe thread. It doesn't drip but it is just enough leakage to stink up the cab. I did empty the tank, disassemble, clean and reassemble, allowing about a week to cure.
Any ideas on how to eliminate this issue?

It may be the threads or it could be the shut-off valve, I would remove the valve and cap the nipple, that way you can determine if the threads are the source of the leak.

The leak is definitely coming from the nipple that threads into the bottom of the tank. Definitely not the valve or where the nipple threads into the valve it is the threads on the tank where the nipple threads into

I would remove the pipe/valve from the tank and thoroughly clean the threads and inspect the threads in the tank and the pipe to ensure that they are clean and undamaged. If the threads are good, reinstall and tighten securely without any sealant on the threads. Clean, undamaged pipe threads are designed to seal without sealant if properly tightened. That is the magic of tapered threads.
I use whatever grease that I have handy on the threads so that I can tighten them tighter than if the threads were dry.

There is a difference between NPT (National Pipe Thread) and NPTF (National Pipe Thread Fuel). Only the latter type is designed to seal without teflon tape or pipe dope. NPTF threads rely on an interference fit (metal-to-metal crush) where crests and roots deform into each other without sealant.Unfortunately it's nearly impossible to tell the difference between them. So definitely use pipe dope (Gasoilia is a recommended brand.) It's been rumored that some reproduction tanks had straight threads rather than tapered threads and are nearly impossible to get sealed. If that's the problem with your tank, you MIGHT be able to run an NPT tap thru it and get a little taper in the threads. You'd still need to use pipe dope to get it to seal.

Well, Kevin, you learn something every day! NPTF.
I looked it up and even AI knew about it.
Makes me feel dumb ! ( with a B )

Yes, I think you hit it as the tank thread looked straight. I will burn off 10 gallons of gas and try your suggestion. I will post the results, might take a few weeks tho.
Thanks for the input

“ It doesn’t drip …” are you certain that the fume source is that bottom tank feed connection? What evidence brought you to that conclusion?

I’m a big fan of Teflon tape, since you are having a issue clean everything with brake clean/alcohol or similar, a small amount of pipe dope (sealant of choice) worked into the root of the male threads, wrap Teflon tape in the correct overlap direction, on a small pipe size no more than 2 tight stretched wraps, a slight layer of sealant over the Teflon which works into the cleaned female threads and assemble. Slight amount means more is not better. No need to crank it excessively, to tight can ruin the tank threads.
As a side note when working we had storage tanks out of Germany that came with straight threads and IPS adapters, we were provided hemp (like dry oakum) to wrap onto the threads for assembly. (PITA? Yes).
Ps: on my tank which I just redid I used an actual pumbing IPS ball valve

Chuck