I always run a tap through all of the threads to clean out the gunk that settles in there over the years and during hot tank... You would be surprised how much crud can get in there...
That's what cooking helps with, crust just falls out everywhere. But yeah decades of that stuff sometimes needs help, pick between cylinders through freeze plug holes etc...When i machined i always ran taps through and i could usually find a bad thread and upsell an insert or helicoil.