I had to tap the holes on a fuel tank that takes that size petcocks (to remove some epoxy primer). I got the tap from British Tools and Fasteners - it is British Pipe Thread, 1/4 X 19 tpi. (Note, it is sized by the i.d. of the pipe.)
correct it is BSP
if i ever need a tap (which is rare cause i have lots left over from my days in the trade) im only going to use a few times . specially for just cleaning out a thread I try the second hand tool places , BSP ( here anyway) is a very common size and i can imagine it isn't that unusual in the USA ? ........they will cost $5 instead of how much new from some bike bits type supplier ?