M14* 1. 50 if you are referring to the injector to high pressure line thread. . (first gen's were smaller)
I've never measured the hold down nut or the threads in the head, but I remember my compression tester being M24*1. 50 for this. .
Assure no fod gets into the injector, I would pull it, turn it upside down in some soft jaws in a vise then put a die to it, you may need a new injector body. Your local injection shop might be able to swap your injector's guts over reasonable. You s/b able to get from one Cummins (recon) for less than a 100. They might ding you on the core, if they don't think they can re-thread it either. .
Good luck
Andy