i just finished re-threading one of my cylinder heads for a 2001 mustang gt, and i was wondering if some of the aluminum shreds that fall can seriously hurt it, because i cleaned it as much as possible but i know there still some pieces there.
Its definitely not a good thing to have metal shavings in the cylinder. If you vacuumed out the best you could, what else can you do? What do you want to hear? Unless you are looking to take the head off there's nothing else you can do.
My brother has had 3 spark plug holes re-threaded on the 5.4 in his truck, while on the motor. We coated the bits in grease first so that the shavings would stick to it and not fall into the motor. We also made sure it was on a compression stroke so we didn't nip one of the valves during the process. I'm sure there still had to be some very small material that got into the cylinder, but never had any problems. Large shavings may create an issue.