We helical interpolate a new repair thread into the head also machining the stub seating face (this face can be worn and if it is, we machine and fit a stainless spacer to recreate a good seat) and then we fit and lock our inserts into the head. These inserts are screwed in and will NOT WORK LOOSE nor can they be jacked out by the exhaust stub. The new thread is then machined into the insert to fit the new stub exactly. Due to the nature of CNC thread milling, the two threads are coincidental. In real terms this provides a very strong and wear resistant high expansion insert…