Tighten a fuel line till there’s resistance. With an adjustable wrench, you turn it another quarter-turn. Fast forward three-hundred miles and the smell of gas drifting through air intake takes away all that confidence. What happened? Most often it was something you did wrong within the initial half-minute of installation.
Chances are, you grabbed the wrong tool and stripped the aluminum hex flat. Or maybe you torqued down entire fitting body rather than just the nut. A single turn of the wrench gashes the delicate 37-degree flare seat inside the fitting. Nothing but a new fitting will do, no matter how much thread tape or added torque you apply. It’s an easy mistake caused from not having the right thing in your hand and being to impatient to use it.
Common Mistakes When Installing Fuel Lines
As you can see on this chart, each common dash line corresponds with specific wrench size. They start out small at AN3 and get huge for heavy duty hydraulic systems up to AN24. Remember the pattern of choosing tools based off those numbers, rather than just the numbers themselves. There is two wrenches needed for each size. You need one to hold the fitting body still and another to turn the nut.
In the world of aerospace plumbing, it doesn’t work any other way. Only turning the nut with a single wrench will cause the entire fitting body to spin within the port. It crushes the flare cone, twists the tube and wrecks the seat long before it’s torqued all the way down. Keeping the body held in place prevents all that. The nut gets turned and pressure is applied where it needs to be.
The tool makes all the difference. Thin jaws of dedicated open-end wrenches slip into confined areas, while standard open-end wrenches (with thicker jaws) rarely do. Better yet are flare nut wrenches that grips five sides of the hex at once to distribute pressure evenly and prevent rounding. Adjustable crescent wrenches must be avoided; they rock on the hex flats and will always cause damage to soft aluminum fittings. Invest in a good set of fixed-size wrenches. Replacing every fitting in your fuel system after rounding them off with an adjustable wrench is more expensive than purchasing a proper set of wrenches.
Know your torque: Steel vs. Aluminum Depending on which side of the equation you’re on (steel or aluminum) the torque specs will be different. Aluminum alloys include 6061-T6 or 2024-T3; they’re soft but lightweight. Too much torque and they’ll strip. Steel is harder; it can takes much more torque. Often almost twice that of aluminum. That’s what the visual guide in the infographic is all about. It illustrates the point well.
Before any torquing begins you’ve got to know what it is first. For example, aluminum is typically blue anodized while steel has a silver and black finish. And a quick magnet test helps, aluminum isn’t magnetic where carbon steels are. Don’t guess, you’ll either crack a flare cone or strip threads.
The Flats From Finger Tight trick works when you don’t have a torque wrench handy. Hand-start the nut all the way home to finger tight. Position the hex so that you see one flat mark then rotate the nut forward one set of flats based on dash size. For most small fittings such as AN3 to AN8 you will only need to rotate the nut one flat. That’s sixty degrees. For larger fittings up to AN24, two full flats is often necessary to get good seating pressure. No more guessing or over tightening! Just go with geometry and forget your muscle memory.
Most folks’ number one error is applying Teflon tape or pipe dope in an attempt to seal the threads of an AN fitting. That simply won’t work; the AN fitting does not seal on the thread. It only seals on the 37-degree flare cone, where the tube connects to body of the fitting. Applying sealant to the threads will cause the nut not to seat fully against the base. In turn, this causes the flare to fail to engage correctly. This results in a loose connection that blows out when under pressure. Keep your threads clean and dry always.
When we tighten things down by feel, it’s tempting to trust our gut. But fuel and hydraulic lines works under pressures that are well above the abilities of our fingertips. Proper wrench size eliminates stripped flats. Back-wrenching keeps the flare seat safe. Knowing what you can and cannot do with materials avoids costly shavings of aluminum. Do it once, do it right. After you close the hood the system will remain dry.
