Sounds like the same size as the drain plug in the bottom of the gas tank. I think David Bell may have some. Ask on the deTomaso list? http://realbig.com/mailman/listinfo/detomaso
Or you could re-tap for a 1/2" pipe thread.
22mm should be .866" but the gas tank plug that I have measures .857"
1/2" pipe nipple measures about .851"
1.5mm pitch is about 17 threads per inch. Pipe thread is 14 threads per inch
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