Heli-coil installation MUST be exactly in line with the old threads of the fuel nut will not clamp the gasket evenly and it will not seal.
It really needs to be done in a Bridgeport or set-up so that you can reference the gasket surface before running the tap into the threads.
I've done several thousand at this point, there is a learning curve involved with this procedure. I'll also add that the only tap that I've seen that works well is from Heli-Coil, and they are difficult to obtain.
I miss-placed mine a few years ago and bought the cheaper one for around $150 and it's a complete POS and chews in and tears up the threads instead of cutting them......Cliff