Teflon tape

[e3steve]

Lee is right on the money; the petcock seals using a flat nitrile washer which sits within a groove in the brass nut. The nut has opposed threading: r/h onto the tank spigot and l/h onto the petcock. Be cautious how much meat you 'face off'; I've needed to use 2 washers on mine, after facing!

I'd avoid using PTFE tape (PolyTetroFluoroEthylene -- or "Plumbers' Tape For Everything!"); it would seal and it is petrol (and diesel) resistant, but it will look like shite and should only really be used in an emergency for this application. The petcock alloy is very old and PTFE tape has a very low friction coefficient (super-slippery); wind up the nut over enough Teflon tape and the petcock thread could implode.

For similar reasons, it should never be used to seal plastic threads!

[waldrtw]

I had some teflon seals here at work that I used and it worked out great. No leaks so far.