In my small experience i`ve found that the Japs are metric, and by golly they love their fine pitch threads. So without looking and i had to hazard a guess, i would say that M10 x 1mm sounds pretty good.
If the thread is at the base make sure you use a bottom tap but if thats the case i don`t understand why the bolt won`t start? Are you sure it isn`t the bolt thats knackered?
Incidentally, if the bolt would have started and you didn`t have a tap but you did have a spare bolt then you can cut a notch down the length of the bolt with a hacksaw and then run the bolt down then this will recut also. Although this should be used as a last ditch effort.