An issue that came up on the Healey email list this past summer eventually revealed a major safety concern with some manufacturers of aftermarket gas tanks, which has prompted this article. It seems that some suppliers decided that an American ¼ inch National Pipe Taper (NPT) outlet thread was "close enough" to the original ¼ inch British Standard Pipe Parallel thread that the NPT fitting was suitable for use on the outlets of the gas tanks they sell. Turns out they were wrong, and evidently many people have been trashing the tank fitting on their rear fuel lines trying to get lines attached to these tanks.

Generally speaking, the threads on systems dealing with low pressure fluids on Healeys—the bits handling fuel, oil, and coolant—are British Standard Pipe Parallel (BSPP). For example, the threads on the dual gauge and for the rocker shaft banjo bolt are 1/8 inch BSPP. The fuel line fittings are ¼ inch BSPP, one size larger, and the seal is made by compressing a 30º nipple into a 30º seat. The NPT fitting, on the other hand, has tapered threads and the seal is made on the threads; when installed, two NPT pieces are twisted together firmly on the taper so that the binding of the threads creates the seal. To quote from the Parker Thread Guide: "a parallel thread form uses the threads for holding power only and will have some other means of sealing [compression sleeve]. A tapered thread may seal by the action of the external threads of one component wedging into the female threads of another component and sealing by thread contact." And the kicker: "Two of the more popular thread forms used in the world today are the British Standard Pipe Parallel (BSPP) and British Standard Pipe Tapered (BSPT) threads. Both of these thread forms have a 55º thread flank angle. (An) NPT/NPTF has a 60º thread form. Therefore, the BSPP or the BSPT cannot be interchanged with NPT/NPTF pipe threads!" In addition, the tip of an NPT elbow is not cut with a 30º seat to accept a compression fitting.

Another very important difference between the NPT and BSPP fittings is the number of threads per inch. A ¼ inch NPT fitting has 18 tpi. A ¼ inch BSPP fitting has 19 tpi, and the extra thread does make a difference. When trying to screw a BSPP fuel line nut onto an NPT fitting you will experience hard-turning and binding on the threads. Since the fuel line nuts are brass and the tank outlets are usually steel the threads of the fuel line nut can be stripped and/or the nut itself split, often without the installer realizing that an unsound fuel plumbing situation has been created. A stripped or split nut may even give the illusion of attaching the fuel line well. Unfortunately, any forced mating of a BSPP nut with an NPT fitting is fundamentally unsound, and the failure of such a union could have severe consequences. At the very least the car will stop if the fuel line parts company with the tank, which might be rather dodgey if you’re in heavy traffic. A Jag owner with an NPT thread outlet tank had the fuel line detach from the tank on his way to the big event in Waynesboro, Va. last October. He managed to get off the road without getting creamed, and was setting out flares when he noticed gas pouring out beneath the car. The Jag guy was able to keep everything from going up in flames, but the whole incident could have been a true disaster.

So how do you tell if your replacement fuel tank has the correct BSPP threads? The easiest way is to check with a thread gauge, but only a Whitworth/BSF thread gauge will have a leaf with 19 threads per inch. Any of the female fuel line nuts, including the carb flexline nut, should turn easily onto the fitting to the stopping point with only firm finger pressure necessary, no tools. Conversely, a reading of 18 tpi with a standard thread gauge, no 30º seat and a noticeable taper to the tank outlet, or difficulty torquing on a fuel line nut by hand could indicate NPT threads. I’d recommend that any owner of an after- market gas tank from the cheaper suppliers check the integrity of the fuel line connection to the tank. Look for brass shavings or distortion of the brass fuel line nut. Try loosening the nut slightly to see if there is any thread binding or wobbling. Then see if the nut can be tightened firmly back down, with no binding or skipping. You can also yank the fuel line forward to see if the connection can fail.

Should you find yourself with one of these tanks using bogus NPT outlet threads all is not lost. The easiest and probably cheapest way to rectify the problem is to fit a ¼ inch NPT to BSPP adapter. McMaster-Carr offers these for about $5 and the part number is 4936K119

I was able to contact two tank manufacturers that were building tanks with bogus outlet threads. The first, when informed of the potential problems and referred to sources for the correct fitting, immediately changed to the correct ¼" BSPP threads. The second, after ignoring numerous emails, finally responded after two months to state that his tanks now had ¼" BSPP outlets. This supplier may have caught some heat from members of the Healey email list who had purchased his tanks and were made aware of the problem by postings to the list. He also was the manufacturer of the tank in the Waynesboro Jag and his notice of using BSPP fittings arrived about ten days after that incident. Both of these suppliers market their products on eBay.