Rajan Wilcox
National Pipe Thread Taper (NPT) is a US standard for tapered threads used on fittings and threaded pipes. NPT sizing designates the I.D. (inside diameter) of the pipe, which is often misunderstood. NPT fittings are commonly used for fuel feed and return lines, with tanks, bolt-in pickups, and fuel pump assemblies. The 5/15 fuel line refers to the size of the hose or pipe that is 5/16 inches in diameter. The NPT size for a 5/15 fuel line is 1/4 NPT, which is a standard size for pipe-to-hose fittings used in fuel systems, especially in boat fuel tank connectors.
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What You'll Learn
NPT sizing
NPT, or National Pipe Taper Fuel, is a US standard for tapered threads used on fittings and threaded pipes. The sizing of NPT fittings can be confusing as it designates the internal diameter (ID) rather than the outer diameter (OD). This means that the actual size of the NPT fitting is larger than its designated size. For example, a 1/4" NPT fitting will have a nominal outer diameter of 0.533", which is considerably larger than a quarter of an inch.
The taper rate for all NPT threads is 1⁄16 (3⁄4 inch per foot) or 1 inch of diameter in 16 inches of length, measured by the change in diameter over distance. This taper allows the larger diameter of the thread body to compress into a smaller diameter, eventually forming a seal. The most common sizes for NPT threads are 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, and 2 inches. Sizes smaller than 1/8 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon due to the use of alternative joining methods for these larger sizes.
It's important to note that NPT sizing is different from AN sizing, which is designated by a dash followed by a number. That number represents the number of 1/16 of an inch of the outer diameter of a hard line. For example, a -6 AN line is 6/16" or 3/8". Adapter fittings for NPT threads are readily available at most automotive or hardware stores, making it easy to adapt from NPT to other line types.
When making connections with NPT fittings, it is recommended to use a lubricant such as lubricating paste or thread seal tape to ensure a leak-free seal and prevent corrosion on the threads. NPTF (National Pipe Taper Fuel) is a variation of NPT that provides a more leak-free seal without the use of PTFE tape or other sealant compounds.
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NPT fittings for boat fuel tank connectors
National Pipe Taper (NPT) is a US standard for tapered threads used on fittings and threaded pipes. NPT fittings are commonly used in boat fuel tank connectors, ensuring a secure and leak-free seal for fuel lines.
When selecting NPT fittings for boat fuel tank connectors, it is important to consider the specific dimensions and requirements of your fuel system. The size of the NPT fitting will depend on the diameter of the fuel line hose and the fuel tank connection. Standard sizes for boat fuel tank connectors include 1/4" NPT and 3/8" NPT, with both male and female threads available.
For example, the Moeller Marine Fuel Tank NPT Connector features a 1/4" male NPT fitting, designed to connect seamlessly with a matching female end on diesel generator fuel tanks. This product has received positive reviews for its quality and effectiveness in creating a secure seal, preventing air from entering the fuel line when the tank is removed.
Another option is the Scepter Universal Marine Tank Quick Connector, which offers a 3/8" female fitting for connection to the fuel line hose and a 1/4" male NPT fitting for attachment to the fuel tank. This universal connector is designed to be compatible with a wide range of marine fuel tanks, providing a quick and secure connection.
It is worth noting that NPT sizing designates the internal diameter (I.D.), which can sometimes be misleading. Therefore, it is crucial to verify the correct size before purchasing or installing NPT fittings. Adapter fittings for NPT threads are readily available at automotive and hardware stores, making it convenient to adapt from NPT to other line types if needed.
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NPT thread sealant
NPT, a US standard for tapered threads used on fittings and threaded pipes, requires the use of thread sealants to create leakproof seals. NPTF connections, on the other hand, are designed to be dry seals and do not generally need thread sealants.
While Teflon tape is commonly used as a packing sealant for NPT threads, it is not a sealant but a lubricant that allows for tighter assembly. Pipe dope is considered superior to Teflon tape as it is easier to use and more reliable. When using pipe dope, ensure that it is compatible with the fluid in the pipes.
For NPT thread sealant applications, Loctite 565 is recommended. It is specifically designed for sealing NPT threads and comes in a white tube with an orange colour. Other options include using the right stuff, silicone, or any pipe sealant.
When reusing NPT fittings, it is important to ensure that no more than the usual amount of thread sealant is used to create a good seal. Overtightening should be avoided as it can permanently distort the tapered threads.
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NPT sizing vs hard lines
National Pipe Thread Taper (NPT) is a US standard for tapered threads used on threaded pipes and fittings. NPT sizing designates the inner diameter (ID) of the line, while hard lines designate the outer diameter (OD). Therefore, NPT sizing is larger than it may seem. For example, a -6 AN line has an outer diameter of 6/16" or 3/8".
NPT fittings are tapered, starting loose and tightening as the fitting is threaded in. This is unlike typical parallel or straight thread fittings, which have a tighter fit. The threads themselves are what seal the fitting, so it is common to see threads showing on NPT fittings and adapters. Teflon tape is recommended to ensure a leak-free seal.
AN thread specification, on the other hand, was derived from the US military and is a standard agreed upon by the Army and Navy (AN). AN fittings are commonly used to connect flexible hoses and have straight threads, using O-ring seals. The O-ring size is typically defined by the "AS" standard. AN sizing is designated by a dash (-) followed by a number, which stands for the number of 1/16 of an inch of the outer diameter of a hard line.
When choosing between NPT and hard line sizing, it is important to consider the specific application and the compatibility of the fittings. Adapter fittings for NPT threads are prevalent and can be easily found at most automotive or hardware stores, making it easy to adapt from NPT to other types of lines.
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NPT adapters
National Pipe Taper (NPT) is a standardized fitting commonly used for joining pipes. NPT sizing designates the I.D. (inner diameter) of the line, which is why NPT sizing is larger than it seems. NPT fittings have tapered threads, which start loose and tighten as the fitting is threaded in, creating a fluid-tight seal. NPT fittings are commonly found on fuel lines, such as the 5/15 fuel line you mentioned.
Adapters are used to convert hose or pipe diameters, such as BSP and NPT threads. There are many different types of NPT adapters available, depending on the specific application and the sizes of the pipes being joined. Here are some examples of NPT adapters:
- NPT x Hose Barb Adapters: These adapters are used to connect NPT fittings to hoses with a barb-type connection.
- JIC x NPT Adapters: These adapters are used to connect NPT fittings to JIC (Joint Industry Council) hydraulic hoses and fittings.
- Tri-Clamp Adapters: These adapters have a tri-clamp design and are used with sanitary fittings, commonly used in industries where maintaining fluid and gas purity is important.
- Rubber Hose Adapters: These adapters are designed to connect NPT fittings to rubber hoses.
- NPS Adapters: These adapters are used to connect NPT fittings to NPS (National Pipe Straight) threads, which have straight threads instead of tapered threads.
When using NPT adapters, it is important to ensure a tight seal to prevent leaks. Teflon tape is often recommended to ensure a leak-free connection. It is also crucial to match the correct adapter size to the NPT fitting, as NPT sizing can be misleading due to its designation of the inner diameter.
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Frequently asked questions
NPT is a U.S. standard for tapered threads used on fittings and threaded pipes. A taper thread starts loose and tightens as the fitting is threaded in, making a fluid-tight seal.
A 5/16 fuel line uses a 1/4 NPT thread.
Teflon tape is recommended to ensure a leak-free seal. Fuel-compatible thread sealant is also recommended to prevent galling and provide the best possible connection.
