Official Luthiers Forum!

Hi Guys, Its been a couple of years since I posted here due to health issues. I was working on a 1950 Gibson CF 100 that I bought that needed some major work (I have a thread of the rebuild here in the forum). It had the typical problem this model was plagued with which is a collapsed top due to being a cutaway acoustic. This was the first and only acoustic cutaway that Gibson made. The basic guitar is a LG-2 with a cutaway.

When I got the guitar, the neck had lifted up and to the right (cutaway side). The top was in bad shape and someone thought it would be a good idea to use lathing (for plaster walls) to reinforce the top and epoxied big chunk of it to the underside of the top. Needless to say the top needed to be replaced ,the neck reset and a number of braces replaced.

When I left off working on it I had the new top on and refinished and was ready to install the bridge and neck. That is when I got sick and stopped doing all work.

So I recently started back up on this and I am having issues trying to set this neck.

First off, When I had the top off I suspected the neck block may have some issues so I paid particular attention to it. All the glue joints appeared to be solid and tight along the back and sides. I wiped it down with mineral spirits to check for any cracks (a crack will stay wet longer than a surface). There did not seem to be any cracks and there were no distortions in the sides and back. So it appeared to be solid and crack free. The back did have 3 braces off so I assumed that took the stress and left the head block intact.

A few weeks ago I decided to get back in the shop and pick up where I left off. The one real issue that I had was someone tried to reset the neck before I got it (and before the top caved in). They clearly didn’t know what they were doing and carved up the tenon. The angle on the left side (looking at the front of the guitar) of the joint had been changed and didn’t match the mortise so I cleaned up the face and shimmed the mortise and proceeded to fit the joint.

I am not a pro but have plenty of experience and have probably done 12 neck resets with no issues until this one (well other than the practice guitars I did to learn on). I have reset this neck 3 times so far. The first time I did it I thought I had the angle right and things seemed to go smoothly. That is until I removed the clamps and started measuring. The joint was over set and was too low… So I steamed off the neck and redid it. I added a bit more angle to it and glued it back up. This time it was pretty good and I (thought) I was happy with it. But when I put the strings on I found it to still be too shallow of an angle. So again I removed the neck and the shims and started over.

So this brings me to this post.

After resetting the joint and putting strings on it was perfect. I had 5/64ths on the bass side and 4/64ths on the treble side. It played really nice and I thought I was done with it. But the next day I picked it up and noticed the action was a bit high and figured it was just settling in. I let it sit for a couple of days and noticed it was even worse. That is when I noticed a gap at the bottom of the heel.

For reference, when I set a neck I chalk the joint and make sure I have at least 90 percent of both sides of the joint mating up. This last time I had 100 per cent of the joint fitting on both sides. I also leave it clamped up for at least 12 hours and do not string it up for 18 to 24 hours.

So my first thought was that I missed a crack in the head block so this morning I pulled the strings and started looking. I wet the block down again with naphtha and could find no traces of any cracks. I prodded the edges with a feeler gauge but it appears to be tight and nothing seems loose.


I am wondering if anyone has had a similar experience and it is something I am not seeing or have run into before. As I said I have done a dozen or so neck resets and up to this point I thought I was pretty good at doing them. This one has tested my confidence...

Any help or input would be welcome.

Thanks,
Bob

Here are some pic's of the inside and out.

Sorry about the HUGE pictures... I resized them down to aprox 1200X 700 but they show up this big.

Bob

A few things could be happening and here's some things you should check.

1. Check the area immediately to the right and left of the fingerboard extension between where it ends and where the rosette begins and look for cracks running parallel to the top. If there's cracks there then the top needs to be repaired/reinforced in that area because it is "slipping" towards the bridge of the guitar. Also check the rosette and look for a little disfigured shape at the top end of it. If it's not perfectly round then you have an idea of the problem.

2. Your dovetail could be loosely fit at the bottom. When this happens the glue will initially hold well and give you a good action. But as the string tension pulls on it the heel moves into an underset position and it gives you the same symptoms as a slipped heel.

3. The heel of the neck block could be loose as well. check the area where the sides and the back meet the neck block and see if there's any distortion.


My money is on the neck block being part of the problem. But from your pictures it really looks like the tenon was not fit tight enough.

@DanKirkland

Sorry I should have given more details about this guitar.

I started a thread here:

https://luthiersforum.com/forum/viewtopic.php?f=10137&t=46629&hilit=+Gibson+CF+100

I replaced the top on this guitar as well as beefed up the transverse brace to keep it from twisting.

I just finished cleaning the mortice really well and removed all the shims. It did have a thick shim on the cutaway side which I am pretty sure was from the factory to flush up the cutaway to the heel. (didn't get a pic of it). it was probably 1/8" thick. I could find no cracks inside the joint but I am going to check it again.

You said:

2. Your dovetail could be loosely fit at the bottom. When this happens the glue will initially hold well and give you a good action. But as the string tension pulls on it the heel moves

That was the original problem, Someone before me shaved the tenon at the wrong angle (not the V but the taper front to back).
I repaired it by making a tapered shim in the mortice. I had almost 100% contact when I chalked the joint but clearly something is not right.

I also think it is something with the neck block but can't see anything yet. Also, there was no distortion anywhere near the top, sides or back.

The only thing I can see is what appears to be a small crack in the finish in the cutaway where it meets the neck. You can see it in the one picture.


Thanks for the reply,

Cheers
Bob

I have checked and double checked for any cracks in the head block but didn't find anything. I know there still could be one there so I did not rule it out, however I did find that the shim that was on the cutaway side (the one that I think is a factory shim to flush up the heel to the cutaway) appeared to be split and quite soft.

So decided to clean up the entire mortice and start fresh. I put a wedge in the mortice to open any crack that might be there and flooded the bottom of the joint with ultra thin CA glue. The thick shim that was split has been replaced with a new maple one and I am in the process of redoing the joint from scratch.

The one spot on the inside of the cutaway that I thought might be a crack didn't appear to be a crack looking at it under a magnifier. I think it was a scratch that may have done. It did get down to the wood so I cleaned it up and drop filled it. I am not being too careful with this guitar as far as scratches and dings since I had to refinish the top and sides and am in the process of distressing it so it doesn't look new compared to the neck and back. And the fact that I bought this for myself.

My thinking is this is probably the best procedure shy of pulling the neck block out, which is something I have not done before so that would be my last option.

I will let you know how it works out.

Cheers,
Bob

Try this before you start doing stuff to the neck block.

You don't have to have the dovetail glued in to apply string pressure. Once you get the new shim in and start getting it fit again then try setting the joint and then stringing the guitar sans any glue. A dovetail is a mechanical join so you should have no issue getting it in and strung up to tension.

Looking back at your photo with the heel separation, the only way that can happen is if the lower half of the dovetail is improperly fit. So a dry string up should tell you exactly what you'd need to know. Try that first before you replace the neck block or anything and see what you can find.

@DanKirkland,

Thanks for the suggestion, I know a tapered dovetail is a self locking joint but never thought to string it up with no glue...

I yesterday I did finish putting new maple shims in and having done that I am pretty confident that it was that original mahogany shim. It was pretty soft and had split apart when I removed it even though I was trying to get it off intact. The crack in it appears to be stained with HHG so I think that was the culprit.

Once I had the new shims in I could tell the joint fit much more solid and tight. I re-flossed the heel and got the neck fitting really well so I glued it up. I just went down and removed the clamps after letting it sit for 18 hours. I will wait a full 24 hours before putting strings on but using a straight edge the neck is exactly where I set it. That was part of the problem before, I set the neck and after removing the clamps it was slightly different than where I set it.

Hopefully this problem is resolved. I will let you know if it stays put when I string it up.

Cheers,
Bob

Lesson Learned...

remove ALL old shims when doing a reset. I am confident the issue was the one shim I did not remove which had a split in it that I did not see.

I glued up the joint and gave it 12 hours with clamps on and a full 48 hours before I put any stress on the joint. Last night I strung the 1 and 6 strings up and checked how the neck was set and it stayed exactly where I set it. This AM I when down and strung it up and set the action and tuned it up. The heel fits perfectly as it should and there are no gaps. It has been tuned up for half the day now and it stayed in tune, no gaps appeared and it plays quite well now. I set the action a bit high to give it time to let it settle in and I will give it a few days and will set the action a bit lower to my preference but at this point I am very pleased how it turned out.

@DanKirkland

Thank you for your help. Just having someone to give me their perspective on this has kept me from getting tunnel vision. Also, I have never really given a thought to string it up with no glue. I have always figured out where it needed to be set and glued it in. Other when I was just learning how to reset a neck I have pretty much gotten it right on my first try up until this one. I have only done a dozen or so resets so I guess I have just been lucky not to have had problems before.

Thanks again for the tip and your insight on this.

Cheers,
Bob

Here are a couple of shots I took of the final set.

Looks good and clean. happy to help where I can