Having cleaned the fuel tank and installed the fuel-level sender, the next step was to pressure test it. The tank has as a lot of TIG welding and if there’s a leak you want to find it before you install it and fill it with gas. I temporarily plumbed a Schrader value into the top left NPT port, a pressure gauge to into the top right NPT port and a ½” NPT to -10 AN fitting with a cap in the bottom port. I pressurized the tank to 5 psi and went to dinner. When I returned the tank was down to 3 psi – I had a slow leak.
To find the leak, I mixed dish soap with a little water and brushed it on all of the seams. Fortunately, none of the tank seams leaked, but all of the NPT ports were leaking. I was surprised and how many bubbles a slow leak produced. I tightened the brass fittings as much as I dared, but they still leaked. While NPT threads are tapered, they don’t have a flare like you’d find in an AN fitting and they are often sealed with Teflon tape. However, Teflon tape isn’t compatible with fuel and if you’re not careful a piece might wind up in a fuel injector. Instead, I bought some Permatex Aviation Form-A-Gasket, applied it to the treads and no leaks! Next step is to install the tank.