The crank should use 9/16" x 20tpi thread; I agree with @
Andrew R Stewart that the crank arm is likely damaged. Is it one side or both? If one side, drive side or non-drive side? Non-drive side would likely be easy to simply replace; if you're not fussy, it doesn't have to be the same make/model, just match the length. Helicoil repairs work, but might be a significant fraction of the cost of a new arm.
N.B. Shimano Dyna-Drive had been out of production for 25 years by the time the OP's bike was made, so I find that an unlikely possibility.