I am sorry Dan, I have looked and cannot find any reference to max purchase in the f-18 rules. Might be a practical limit rather than an actual rule.

Ok andrew just for you, some close up pics...

you can see the core exits the outer about 5 inches from the end, the outer is then through the core. so basically both the outer and the inner pass though each other... then the remaining outer is buried inside the core. the constriction force of the core wrapped around the out will make it a very strong joint.

Dont pay some one to do it, gets some fids and do it yourself, its pretty easy once you get the hang of it.