Sealing Fuel Fitting Threads
A frequently asked question from DIY boaters is how to seal the pipe threads found on fuel system components. For most components such as a pre-fuel filter (aka fuel water separator) the thread type is typically NPT or NPTF and requires sealant around the threads. NPT is a tapered thread common to plumbing connections in North America, and is also the most common thread type in fuel line components. It is critical that you first know the thread type in the component you are purchasing and then mate it with the matching thread type. Most fuel filters, pumps, tank connections, etc., come with female NPT or NPTF ports. To plumb a fuel hose line you must assure the thread type and if NPT or NPTF then use a Male hose barb fitting. Brass or stainless steel is the the go to metal for these components. The matching thread keys into the female thread precisely both on the angle and depth of thread. NPT leaves a minute gap at the very point of the male thread tip and female valley. [see Pipe Thread Standards Explained.] Another thread type sometimes used is UNF such as on the Racor Turbine Fuel Filter series. This thread form is straight and requires the matching UNF thread to mate with. Both of these thread types are designed to work with a sealant.
How to choose thread sealant for fuel applications
To choose the appropriate thread sealant, look for its chemical resistance to fuel (diesel or gasoline) or oil running through the line. Backyard mechanics will tell you they've used regular old gas resistant teflon tape (yellow stuff) with success. This is not recommended because it risks breaking free. Liquid or paste type thread sealants are kinder to sensitive components down stream. Mechanics repairing injectors, fuel pumps and carburetors can tell you firsthand it is not uncommon to trace fault in the fuel component to clogging from a piece of teflon tape that washed off the ends of the threads, clogging in the first tiny micron orifice it encounters, if not the fuel pump then an injector or carburetor gallery. Either way, it's a costly repair. Liquid or paste type sealant won't clog. For gasoline, regular old gasoline resistant Aviation Form-a-gasket Number 3 is the best option. Another product also offered by Permatex is the High Temperature Thread Sealant. Tech data on this sealant says it has medium solvent resistance and is recommended for fuel sender type applications. With the advent of ethanol present in modern gasoline, try Form-a-gasket No. 3, which specifically states solvent resistance to gasoline, especially on modern engines that use injectors rather than carburetors such as the E-TEC and HPDI (high pressure direct injection) and any diesel engine.
Whenever making these connections from a threaded port to a hose barb connection, ABYC guidelines call for two hose clamps at each connection point of fuel line over hose barb. Be sure the clamps are a marine-grade stainless steel type with even sealing tension. Doubling up the clamps does requires a hose barb long enough to fit both clamps. If you're installing fuel tanks or repowering a vessel, be sure to consult a professional or, at minimum, have your finished work inspected to be sure there are no siphoning and fuel leak hazards. It's a good idea to have the system pressure tested by a professional anytime you make alterations to a fuel line assembly.
Fuel Hose Ratings: What type of fuel hose do I need?
Whenever selecting fuel hose type, first assess where the fuel line run will be located. Of primary consideration here is above or below deck. Above deck means in the open (such as an outboard engine connected to an above deck tank). Below decks not only includes below deck areas but also hoses anywhere running through confined compartments. There are four grades of fuel line: A1, A2, B1 and B2. You MUST use A1 fuel hose below deck on the high pressure side (after pump). A1 fuel hose offers the highest fire resistance and the least amount of permeability, in other words A1 is your best choice. A2 has less fire resistance. B type fuel hose is more permeable and used more for less volatile fuels like diesel applications. B1 again has higher fire resistance than B2. If you are in doubt as to which fuel hose is appropriate where, or if you're going to keep fuel hose on the shelf, A1 is the way to go because it works everywhere.
Fuel components are no laughing matter - every year boat fires occur from faulty installations. If you're not confident in your installation know-how, it's best for your own safety and the safety of all your passengers to seek help from a certified marine professional. If you've read this far, you're one step closer to a smooth running engine.