Originally Posted by MotorBicycleRacing
The thread is 10 mm 1.0 on the China Girl kit tanks.
Listen to the man. It is a machine thread, not a tapered NPT pipe thread. It needs a washer to seal.
If you're careful you can run a 1/8" pipe tap into the threads and a standard NPT (National Pipe Thread) petcock will thread into it. The trick is to not run the tap in too deep.
Your other option is to buy a petcock from a vendor who sells the Chinese kits. It still will require a gasket, typically the dime sized red one that comes in the kits, to seal the petcock against the tank.
Do not rely on teflon tape or thread sealing compound for a leakproof seal. And please don't use any glue like JB Weld.