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I have used the Cumpiano barrel nut method since day one back in 2003. I originally did not use reinforcement, only flooded the area with thin CA. (If I held the neck facing down while doing it did not wick into the heel)

I later did some pull out testing (published here many years ago) and found the wood cracked in tension at a fairly light load. I first used carbon bar reinforcement and the testing jig broke before the tenon failed. Problem was I put the carbon inserts in before drilling the holes for the barrel nuts and the carbon fiber really dulled the drill bits.

I switched to Maple reinforcement. Testing showed it was just as strong and it didn't damage my drill bits. The inserts run back to about the midpoint of the barrel nut holes. I have a router jig I can clamp the neck in and rout the slots for the inserts.

I was not aware of any of my original un-reinforced guitars failing but Interestingly I recently saw the very first guitar I used the Cumpanio method on (2003) for a broken headstock and on taking the neck off saw that there was a fatigue crack in the lower bolt hole but it had not fully separated.

Tenon Jig II by Terence Kennedy, on Flickr

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It's not what you don't know that hurts you, it's what you do know that's wrong.

In that big hole? A spring and another bolt. It's a rather complicated fully adjustable neck I made for an OM. The whole saga is on 'another forum'. This might explain it:

I'm building a simple adjustable neck that was on an 1829 Stauffer. It has a pivot point, a dowel, and a bolt. I don't believe it was meant to be adjusted all the time, or on the fly. More like what one might do with a truss rod. I don't know, my arch top stays the same.

I was going to use a threaded brass insert; I have a bag of them! The insert would be in the block, and not a narrow tenon, so it doesn't seem like it would have a splitting problem. After seeing this post, I see that method with the barrel nut. It seems like a good idea, but would the brass inserts really cause stress cracks?

I see one potential problem; since the barrel nut would be IN the block, out of reach, I'd have to be sure that the nut was very snug in the block so it didn't twist when the neck bolt is taken out!

Dave; that spring, and engineered support for the back looks very high tech.

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Why be normal?

Ken, I wouldn't worry about the insert in your application at all, installed from the inside it isn't going anywhere.

The thing about most adjustable neck joints that causes me some concern is the pivot point that is acting like a fulcrum and kind of like putting a stick across your knee.

But looking at your sketch, it occurred to me with external or internal or a combo of both fastener types, you could take the neck off and put a flat wedge in there like they sell for electric neck setting. That would take the stress away from the fulcrum and spreading it along the whole heel.

As you say, it isn't something you need to do often or maybe ever.

I think I'm going to have to revisit the idea of an adjustable neck again.

My fingerboards already extend over the top.

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Brian R, Wood Mechanic
N8ZED