Water jacket rust on bare block

413 said
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 ere 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.

I gave away my thread chasers years ago because they were worthless.

The reason the tap takes out metal is because the hole is no longer round from being pulled on and the screw load applied to the thread. A chaser can’t get the hole back to round.

If you break a tap off in a hole that’s on the end user.

The right tool for the job gets the hole back to round and should cut the threads to the bottom of the hole. A chaser won’t do that either.