Hi, Si   A few pictures are worth a thousand Andyjr1515 words.   For Happy Jack's Lull, I checked the bolt length and insert length first to make sure that the bolt would use all the threads in the insert:     So that also gives me how deep a hole in the neck heel I need so that the bolt doesn't bottom before it's fully tight.   Machine bolts don't have the 'flex' that wood screws have - the inserts have to be fully in line with the hole.