[sperry]

My big concern is that w/o NPT threading, you're never going to get that bolt tight enough in the hand-tapped threaded cast iron to have a washer seal from exhaust leaks. Even if you do get it tight initially, the expansion difference between cast iron and grade 8 stainless will eventually loosen that bolt.

There's a real reason EGT probes use pipe-thread. I'd pay the $15 or whatever it costs to get the hole welded... actually, I'd pay the $25 or whatever to get a real bung welded on there and then just put the correct plug in the hole if I wasn't running the EGT sensor and leave myself the option of putting an EGT in there painlessly in the future.