The size is 10mm x 26tpi but a 10 x 1.0 tap is close enough (25.4) I recall the official Tap (Campagnolo?) was $25 back in the day but you could find a 10mm spark plug tap at most any hardware store etc for just a couple bucks and that will work fine. Downside of any 'thread cutting' tap is just that it cuts the thread so in multiple use situations you will end up with less that was there before. Notching a tool or even a 10 x 1.0 bolt might be better but best would be a 'full roll ' tap which reshapes and does not remove any material. This would be for damaged threads if you were converting an non threaded dropout a thread cutting tap would be required. I have seen thread files with a threaded end to repair inside threads but have never seen anything adjustable like we see for outside threads. Anyone who has knowledge of such an item I would like to see it
BOB FREITAS
Sunny MILL VALLEY, CA USA