Vertical Sliding Dovetail / Locking Dado Neck Reset - How to keep joint and change angle?

[Chris N]

This is my first post/discussion.  I'm a new player and fixer-hobbyist who's trying to resuscitate a '70s Conn Student-grade Martin knock-off OM that may have been made by Matsomoku.  I'm looking for some guidance here as the next step after not getting much response to the query at AGF http://www.acousticguitarforum.com/forums/showthread.php?t=484657

Neck is off and joint is not the doweled butt joint I expected but a sliding dovetail (or locking dado). Pics -

[IMG]https://c1.staticflickr.com/5/4463/37173411160_5a3e425eb3_b.jpg[/IMG]

[IMG]https://c1.staticflickr.com/5/4408/37383421896_df18fecaaa_b.jpg[/IMG]

My goal is to reduce the heel for proper neck angle and leave the dovetail in place while adding a bolt-on conversion for ease of future work. There is already a factory installed bolt at the heel's bottom that they used to hold the heel in place while the glue dried, so with the kit, there'd be 3 bolts total.

My question at this point:

1. How best to modify the tail and/or socket so that they allow the neck to be pulled back once the heel's reduced, while maintaining the design's integrity? So far as I can tell, the tail's part of the neck (true?), so it will not permit the neck to be pulled back without excess tension on everything, and I don't want to force/stress it.

To allow the neck to angle back, it would seem necessary to cut a slot at the top (neck side) of the tail down to the fretboard's bottom that would effectively severe the tail from the neck, but there's got to be a more pro way to handle this issue.

Alternatively, it might work to reduce the thickness of the wide part of the neck-end of the tail by some amount, with a gradual tapering of the reduction to zero at some point on the tail as it approaches the soundhole end. That should allow the neck to rotate back a bit. I can't tell if material would also need to be removed from the slot to allow the needed forward movement of the tail's bottom to avoid stress on the joint from the rotation.  The joint would necessarily be a looser fit after that adjustment.  Also, modifying the tail to allow the neck to rotate back might mean the fretboard extension rotates up, leading to tension when I try to glue it down.

I don't want to change the scale in any way, or impact the neck alignment the dovetail provides. I can't find anything online about resetting this neck joint, which suggests it wasn't intended to be reset - challenge accepted.

Is there a better way to reset this neck?  Am I on the right track?  Has anyone here reset a neck with one of these joints?  The pictures I find online showing this joint are the ones I posted.

Thanks for any guidance.

[Mike Semenock]

Thanks again, Chris. Very helpful. It seems I've seen other Conn neck joints on the web just lately. Where they were hiding, I don't know. I thought about getting the joint X-rayed, but blew the idea off until I saw such X-rays a guy had done by his veterinarian! I may just see if I can get that done.

Back to one of my questions; the truss rod end must be captured in the heel because it doesn't show in your photos, or any other shots I've seen for that matter. Makes sense, but I thought I'd see the end of the truss rod.

Seems like I could get the neck reset done. I'll probably try it on a junk Guitar or two just for experience. No guts, no glory. Even if I fry the Conn, but I doubt it; the gold plated Grover tuners are worth $50. Never mind that. I'm keeping a positive attitude!

Also thanks to Jeffrey Suits for some great input to this thread.

Thanks again,

Mike 

[Chris N]

Re:  Truss rod end - I don't recall seeing it exposed, so I'm guessing it's  buried under the fretboard and in the joint base.  That was another concern I had about simply doing a bolt-on neck by sawing it off - what happens to the truss rod when you're done?  Or does the cut avoid the truss rod?  I don't know.

Apart from the joint, the reset requires you to use tried/true formulae to calculate how much of the heel to remove.  Keep in mind you'll be removing a little wedge with the thickest part at the back, and getting thinner as you move to the fretboard.  You have to calculate the wedge size as a function of your desired action range and anticipated saddle height.  I forget what I did, but I had to calculate so as to allow me the maximum safe saddle height (for tone) with my preferred low action (4/64 on E, 3/64 on e).  It worked out great.  Do understand that process, though!

Jeff Suits was also helpful to me in tuning my Bridge Doctor for best tone.  Thanks Jeff.

[Mike Semenock]

Thanks. I'm aware of the calculators online for the heel angle. I do probably too much research into guitar mechanics and repair, and not enough practicing/playing. Just how my mind works.

[Chris N]

Gee, I can't relate to that, AT ALL [insert sarcasm emoji].  Nope, I do the same thing, but I've always been attracted to the mechanical aspects of things - guitars just happen to provide multiple benefits, rather the narrower range I'm used to.  I'm a recovering motorcycle addict, as well, and I always did way more wrenching, than riding.

[Mike Semenock]

Chris,

Did you do any shimming because of the neck angle change or the distance from the saddle due to the neck pivoting on the bottom of the heel away from the saddle?

Thanks,

Mike

[Chris N]

For some reason, I answered this question in a different response (8:07 pm), but here's what I said (it remains the best I can do on this one):

"As for your shimming question - I took material away from the heel bottom (where the heel meets the guitar top), so no shimming involved.  I removed material there, then notched the tenon to allow the neck to come back a bit while remaining largely intact.  I may have misunderstood your question re: shimming, however."

While I did not shim any part of the mortise/tenon, I did, as mentioned elsewhere, add a little shim material (mahogany) to the guitar's top due to a little tearout from when the neck came off.

[Mike Semenock]

Thanks, Chris, for getting back to me on this after two weeks or more. I appreciate it.

Mike