Locking gas cap

I have a skyhawk GT2a. It has a gas cap like this one http://www.monsterscooterparts.com/fueltankcap.html

I want to find one that locks though. I don't know the threads dimensions though.

And my bike is coming out of the shop today, so I can work on it.

It threads on like a water bottle cap... It's different than the regular-bolt on gas tanks. Also my mechanic told me the rear axle pin is too short... Called piston bikes and asked them how they fit theirs on their complete... And they said they just torque down the frames so I picked up a 7" axle. Orig. was like 5.5"