Good thread. Here's an alternative to using the thicker gauge lock ring:
I recently removed a non-leaking sender to make an adjustment on the float arm. After re-installing the gasket it leaked with a full tank. Took a close look at how the ring was seated and it was actually the rubber gasket that had moved out of position. There is a narrow sealing ring on the tank, and while twisting the lock ring the gasket can move off-center.
After re-positioning the gasket and carefully seating the ring, the same gasket did not leak.
The gasket which came with my Spectra tank had a thickness of 2mm or 5/64". I found a 12"x12" sheet of 3/32" nitrile or Buna-N (which has good fuel resistance), which is approx. 2.4mm. Cost $10 incl. shipping. You can cut your own slightly over-size gasket which will not slop around as much during assembly, and the extra 0.4mm will help with the seal-off.
Also, here is the amount I bent the float arm in my 67 Coronet (previously it was straight). At 1/8 reading on the fuel gauge it took 13 gal to fill, meaning there was approx. 6 gal left. This calibration took 3 attempts including the familiar leaking gasket ordeal, so maybe this will help others.
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