Hope you're all doing alright and keeping busy! I've just started taking in a few repairs this week in a safe way: it's nice to have instruments on the bench again!
One is a Hofner Senator which has a problem I'm sure many of you have seen before in Hofner archtops: failing neck joints.
When I got it, the neck was falling out of the dovetail mortise. It's a very shallow angled dovetail with almost no mechanical strength. I'm planning on bolting it back as the customer doesn't want to pay for the additional time required to recut the dovetail.
The other problem is the joint between heel and neck. It had been reglued, badly, so I steamed it apart. I'm looking for a good way to join these two parts to make it a more stable joint. I've a few ideas, but wonder if anyone's tackled this before, or if you might have better ideas than me.
The neck joint isn't exactly a dovetail even though the sides are slightly flared. Doesn't really relate to your question but reminded me of an 8 page thread over at the Maestronet Forum.
This type of joint is typical of viol family instruments. The strength all comes from the sides of the neck heel and the faces of the mortise, they must be a perfect a fit as possible. It is also important that the bottom of the neck be fitted flat down along it's length into the mortise. A glued end grain doesn't doesn't doesn't contribute much strength but eliminating any gap between the heel and bottom of the mortise does give it a solid footprint in the joint, which helps mitigate rotational stressing of string pull. Viol family instruments use an an extended part of the back as a heel cap, the button, which substantially adds to the joints ability to resist string pull, the Hofner joint does not take advantage of this. Hot hide glue should be used for this type of joint.
Your plan to convert the guitar to a bolt on will eliminate the fussy nature of this type of joint and make any future neck adjustments a lot easier.
I would abandon the dowels and cut a vertical spline slot(s) in both parts for a carbon fiber spline(s) and glue it up with West System epoxy.
One advantage of the carbon fiber used with epoxy, is the spline fit can be somewhat loose and allow some for adjustment/ alignment of the neck heel parts. Epoxy still has great gap filling bonding strength with joints that are not tight. One could use wood for this but I would have more confidence that the carbon fiber would be more enduring.
Many of the old Hofners actually had a bolt on neck. The bolt would be under the strap button on the heel.
However, it doesn't seem to be the case here.