Tony,
You cant just tap it, has to be drilled.
Just had a chat with my brother who is an aircraft engineer for flybe, before that he used to work on piston engined prop planes, he has done this job all the time.
They do it with the head on, but the exhaust removed so you can suck the swarf out as its produced, and then flush it out after, any tiny bit of dust on (if its alloy) apparently get burned up. He reckons most car places do it without removing anything and just suck the swarf out..
He was keen on doing it for you in return for some jedi training credit but he's in Cov and hasn't got time to go to MK…