Rectangular port 188's on a 454. The heads are on the block. I'm concerned about crud and maybe a some metal shavings where they might end up (I am using a true thread chaser kit, not a tap, but I still think some metal may be cut out with the chaser). I see that some of the holes have passages past the threads. I can use grease on the thread chaser but wonder if I'm doing more harm than good. So should I chase the threads in the head or leave them be?