What do you use on the threads of the fitting that screws into the gas tank? Is the Teflon pipe tape you see for home plumbing gasoline resistant? The fitting that screws into my gas tank is leaking around the threads. I can't tighten it enough to stop the leak. I am guessing I need some sort of pipe dope or pipe tape on the threads?
Pipe threads are tapered, the design is supposed to get tighter and seal better as you thread it farther into the fitting, some type of sealer is usually needed unless the fittings are brass. Brass is soft and the threads will form pretty well and if the joint is low pressure you may not need sealer. IF the fittings are not pipe thread, such as a flare or inverted flare, the threads do not have any sealing ability, it is the face of the flared pipe against the flare seat that seals. I have used teflon tape sparingly on auto pipe threads, I also have used a gasket sealer such as Permatex Motoseal sparingly on difficult leakes. If your fittings are pipe thread then teflon tape should take care of it, If the joint is in good condition, normal white tape should work, it is thin and fills the microscopic voids in the threads. If the threads in the tank are good, you may have to use a new fitting so you aren't working with worn-out deformed threads on that half of the joint.
Last edited by 78buckshot; 11/07/20235:23 PM.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
It combines blue thread locker with thread sealer.
1950 Chevrolet 3100 (Ol' Roy) 1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1956 Cadillac Sixty Special Fleetwood (The Godfather) | 1966 Oldsmobile Toronado (The Purple Knif) | 1966 Ford Mustang (Little Red) | 1964 Ford Galaxie 500 coupe | 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
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1952 Chevrolet 3100, Three on the Tree, 4:11 torque tube Updated to: ‘59 235 w/hydraulic lifters, 12v w/alternator, HEI, PCV and Power front Disc Brakes Project Journals Stovebolt Gallery Forum
If the threads on the tank and the brass pipe are in good shape, you can put axle grease on the threads which will allow the tapered threads to mesh together to make a perfect seal. The grease won't seal it, it just reduces friction so that the parts will do what they are supposed to do with less force applied. You should use a small pipe wrench for this pipe. Curved-jawed vice grips designed for pipes will work too.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
NPT threads are designed to be used with some type of sealant. When a metal to metal seal is required NPTF threads are used. What's the difference?
NPT (National Pipe Taper) is a general-purpose tapered pipe thread, and NPTF (National Pipe Taper Fuel) is a dryseal tapered pipe thread. Visually, NPT and NPTF pipe threads look identical. Both thread types rely on thread interference for tight, robust connections. This thread interference is made possible primarily by their tapered design.
The two thread standards differ in how they create leakproof seals. Both require proper installation but leakproof NPT connections require the appropriate use of thread sealants. Correctly assembling NPTF connections create leakproof seals without the use of a sealant.
NPTF threads are used instead of NPT when:
The use of sealants could contaminate the fluids or gasses flowing through the system Applications are critical or the thread sealant might fail because of high heat or high pressure
There are important issues regarding the installation, mixed-use, and reuse of NPT and NPT fittings:
NPT and NPTF fittings can be mixed but this requires using thread sealants NPT and sometimes NPTF fittings can be reused but only under particular conditions and cautions NPT and NPTF fittings both have special installation and assembly requirements for leakproof seals
The difference is the clearance at the crest and root of the threads. NPT threads have a little clearance that would need to be filled with thread compound or teflon tape, while NPTF threads have none. See the pic.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Good info, but how do you tell the difference between NPT and NPTF when you look at them? Are they stamped?
BTW, I have heard that teflon tape is not to be used on our trucks' fuel systems; use dope only. There is a yellow teflon tape for gas such as natural gas or propane.
1. All that goop is too goopy and messy. 2. There is absolutely NOTHING wrong with non goopy Teflon white tape. 3. It does not desolve in gasoline. 4. It's simple 5. It seals 6. If wound in the correct direction, not over applied and a little space left at bottom of threads, it's the best choice for non goopers. Use it. 7. PTFE (DuPont brand name: teflon) is non soluable in about every substance/chemical/solvent on earth. 8. Goop can get into any place you think teflon tape can get into. Don't finish the "tag end" at the bottom of the thread. Start the wrap several threads up from bottom and wrap upwards away from bottom of threads so as not to shear tape at bottom of threads. 9. Always cut tape with scissors to make a clean non stringy end, on both ends. 10. House plumbing may have a different set of methods/opinions.
