Now that I have hrassed you into posting this on the forum instead of emailing me directly, I am not sure I can answer your question directly.
I am the proud owner of two tap and die sets, one sae and one metric. The kind that costs about $12.00 So I may have re-cut the thread on the brass fitting that screwed into the tank but I do not recall having to do that. The Motobecane gas tank has a square nut on the inside of the tank, held in place by metal tabs, the petcock screws into this nut and a washer between the petcock and the outside of the tank seals it, so I can't measure the thread without draining the tank and un-screwing the petcock.
My best recollection is that the nut had a standard thread which either matched, or was very close to, the thread on the brass fitting I used. I may, or may not have run a die down the brass fitting to make it fit but I just can't remember. I am sure that I had no real trouble maked it fit.
I hope this helps.