Hi, have just had some work done on a S/Rocket head, one of the repairs was on a thread insert fitted incorrectly. Whilst there, the cyl. head guy (40 years experience) showed me the 3 types of insert, call them stage1,2 and 3. Stage 1 is a helicoil for cross/stripped plug threads. Stage 2 is a copper coated medium steel insert for use where there is insufficient metal to accept a helicoil. Stage 3 is a heavy duty copper coated steel insert for totally butchered holes. These have to be fitted by an skilled engineer. mine wasn't and came out attached to the plug whilst removing it for inspection. This was remedied by using a special loctite. They told me if the insert ever needed to be removed, it would have to be machined out, it's that good. Don't know your location, but the firm i used was South Cerney Engineering Ltd, Ashton Keynes near Cirencester. Worth checking out their website.