If you can't get a thread gauge inside the holes, first measure the ID of one using a calipers, and then find/whittle a stick to slightly over the diameter and screw it into the hole far enough to get as many threads as possible, the unscrew and measure the thread count from the stick. Even a fine ruler should give a decently accurate measurement, but since good tools are always needed and both the thread gauge and calipers are very reasonably priced, buy them if you don't already have any. An old copy of Machinery's Handbook, gotten online cheaply, is also worth having. This looks like a decent copy which is new enough to have mostly current information:
I doubt IH used any metric bolts even on flywheels of vehicles sent overseas, 'course, I can't say for sure...but still I'd bet it should be 3/8-24. If I were in your place, I'd double-check that size again.
Last edited: Aug 10, 2019