Petcock leaking on 1978

Gas leaking from bolt. How can I fix this? I cant find any replace parts for it. Should i use lock tight to sealed it? 

Depending on what 78 thing we are discussing, there is likely supposed to be a nylon or copper washer under the bolt head that may be missing, cracked or badly deformed, thus causing the leak.

The leak could be somewhere else, like the big oring, and running down to the bolt head and dripping off from there.

Forgot to add the Kawasaki 650kz! hahahha! 

You can use blue hylomor sealant on the threads. It’s gas resistant and doesn’t set hard

Kawasaki part# 92022-183, plastic. This is just one part number, they went thru a couple different materials/thicknesses thru the years. I like the old fibre ones better, plastic can split. Not a big deal though and you rarely run across one that did. Red steel, fibre or plastic are all 6mm ID and will work. Wookies concept IS correct but You have a tank to petcock O-Ring without the "Strengthening Sealing" bars built-into the oring. Just a simple 2.5mm preformed oring. What happens in this case merely sealing the threads won't fix a leak because fuel is present because of the petcock filter tower. So fuel is constantly flooding ALL areas within the perimeter of tank to petcock oring. A sealing M6 washer, not some generic steel plain, is mandatory in this case. Have fun! 

Because pictures speak better then babble, check-out the options and what i was talking about mentioning those "Strengthening sealing" bars molded into oring. Yours does not have that. Some pictures of the various options, red steel/fibre and plastic. But you can see fuel is not only possibly weeping from the tank threads, well look and it'll make sense. You could even go to your local lawnmower shop and buy a Honda GX bowl/drain gasket, still red steel M6 gasket you seek and it'll work just fine although thinner and a smaller OD then original. Just make sure you use a nice smooth flanged screw/bolt to whatever length required. I do a lot of the oil injection pumps on Suzuki/Kawasaki/Yamaha and that's why i mentioned the propensity of those plastic washers cracking in operation. I replace with what they call Stat-O-Seals but that's a whole different subject. There's Your ham sandwich, success be Yours!