Water jacket rust on bare block

A life long machinist is a special breed. They can do things a hobbyist cannot. And they have the “feel” for things.

So for the hobbyist I recommend thread chasers. It will keep you out of trouble.

I’ve never broken off a tap in a hole. No reason to go that far, it will tell there is a problem, but you have to listen.

Use a thread chaser in one hole, and a tap in the next hole. Then compare the amount of shavings you have. We did this in school to prove it to the non believers. Use The right tool for the job.