Teflon tape. Works every time for me, just trim off the excess with a sharp knife after the parts are fitted. Also available as a paste, and it works OK too. Easy Peasy to clean off the used teflon when disassembled, just pick the threads off with your finger tips or tweezers. Then there is gasket sealer lacquer, but it leaves a tacky residue when you disassemble the parts later on. The residue comes off with acetone. Both are impervious to petrol, oil and water.
I was fortunate to purchase a small stock of these copper/neoprene petcock washers from Armours when they gave up selling this stuff. They make a perfect seal as the copper squeezes up on the rubber and they give you the chance to put the tap exactly where you want it. https://www.kjixdo.com/index.php?main_page=product_info&products_id=647073
Otherwise the next best thing is "Dowty Washers" which are ally with a rubber insert. Ron
I just use plumber's tape which I suppose is Teflon. Works fine.
Ron, those washers look good but at 10 quid plus postage they'd need to be. Do you reckon they're worth it? I see your point about being able to line up the taps.
They are expensive Bob. Which is why I was lucky to get some for nothing when Armours got rid of their accessories stock. The Dowty washers are similar at only 25p each. Ron
Thanks for the help Ron.
Unfortunately Amazon UK won't dispatch them to Oz for some reason & all I can find locally is still pricey. I may have to reflect that what's on the bike at the moment works...
I use PTFE tape and Loctite flange sealant under the washer. I leave it overnight before putting petrol in the tank to allow the flange sealant to do its thing......Works well..I use the same method on the crank case breather thread, the engine oil unions into the crank case and flange sealant only on the oil tell tale blanking plug into the crank case....Ian