Thanks for the answers. I chased it through with a 7/32 whit which is just a bit sloppy. I have just checked my stock of pre war Govt. surplus taps with a magnifying glass and my thanks to Tman, the carb is indeed a 1/4 whit thread. I need to take more care judging sizes by eye. goodness knows what the army ever used some of these strange sizes for!
