Why does my petcock keep leaking?

[mplecha]

This is driving me nuts. I know this should not be this hard. I have a 78 FLH. My old petcock came apart one afternoon on my ride home from work. I put a spare on, and it leaked. I bought a pingel, and it leaks. It's not the petcock itself - they're leaking up the threads on the tank bung. I've tried permatex, yellow teflon tape, and even nothing on the threads. The same thing happens every time. It seals up fine when I put it together and leaks after the first ride. Is it the vibration? Am I over tightening? I have some loctite 565, but I haven't used it yet. I don't think it should need any sealer, and the others haven't worked in the past. Any advice?

[Hossamania]

Check for a crack or pinhole at the bung weld.

[a_disalvo]

If it's the Pingle, you can send it back to them and they will restore it for a small fee!! Check that equal amounts of thread go into the petcock. Is the end of the threads on the tank square without any burrs?  Frank

[Ohio HD]

I have seen the threaded bung on the tank split from being either over-tightened or was maybe struck by something.

[crock]

I had terrible problems with that lefty/righty petcock and found the tank bung hole just the right size to tap 3/8 pipe threads. Bought the correct petcock and never looked back
https://www.amazon.com/Motorcycle-Filtered-Petcock-Straight-Outlet/dp/B06XSKG453/ref=sr_1_15?ie=UTF8&qid=1538535432&sr=8-15&keywords=harley+tank+petcock

[tmwmoose]

If all else fails look farther up the gas tank on the inside towards the back mounting bolt area . You may have cracked the tank wall (common) and the gas runs right down to the petcock

[david lee]

If all else fails look farther up the gas tank on the inside towards the back mounting bolt area . You may have cracked the tank wall (common) and the gas runs right down to the petcock

yes,i forgot about that happening

[mplecha]

I don't think the wall is cracked. There is some staining there but very light. I removed and reinstalled it yesterday and left it full of water. I checked it this morning, and all is dry. Hopefully it will stay that way. Thanks.