Tank leak fix
A while back, I bought a basket case bike off craigslist. Among other things, the tank studs were leaking, so I JBWelded them. Bought a new petcock, too.
The JBWeld seems to be holding, but the tank was leaking at the petcock. I threw my hands up in disgust and just ordered a new tank and another petcock, which should arrive next week.
In the meantime, I remembered that the pipe fitting on the tank where the petcock goes was rounded at the edge instead of flat. I decided to file it flat to see if that would help. After filing, I screwed the petcock back on.
The problem was obvious: one side of the gasket got snug before the other. The threads in the pipe had been made cockeyed! I filed the high edge down and tightened the petcock again. Filled the tank with water to test for leaks, and found none.
If you have this condition, you may notice that tightening down the petcock causes the gasket to deform. Check your tolerances, and grab a file.