The thread size is M14 x 1.25. They are a standard ISO metric thread form, but the hole threads have a tighter tolerance than you would see for most fasteners. It could be that a regular M14 x 1.25 tap (if there is such a thing) would cut the treads a little deeper into the sides of the hole, making the spark plug fit looser than is desirable. for this reason, I would use a tap specifically for re-threading (chasing) spark plug hole threads. Amazon has a bunch of these. I haven't looked for this at an auto parts store, but I would think they would carry one.