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Thread: Body movement in a 1915 A.

  1. #1
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    Default Body movement in a 1915 A.

    I want to replace the fingerboard, add a carbon fiber neck rod, and refret my 1915 Gibson A mandolin. With the board off, I noticed the neck had slipped .012" downward in relation to the level of the top.
    I also have separation between the neck block and the top edge of the instrument's sides. I'm afraid that tightening these separations would exacerbate the neck movement problem.
    The back has also separated on the treble side near the tail, where the side bulges out with respect to the back. I'm not sure if I should persuade the side back inline.
    Also, I'm wondering what is the translucent amber layer between the neck block and the top. I need to replace some of it. Thanks.
    I need to apologize because I already posted these questions recently onto my November 4 “Truss rod advice” thread, but I'm starting this separate thread now because I think they are separate questions.
    I'm trying to get a system-wide grasp here. ...Harmon Gladding

  2. #2
    Registered User sunburst's Avatar
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    Default Re: Body movement in a 1915 A.

    That's a lot of questions all at once. I'll see how much of it I can cover without investing too much time when I, of course, should be doing something productive... (yeah, right...).

    I'm not sure what you mean about the neck. The fingerboard surface is .012" below the level of the top? I don't see how it could have slipped and done that. If I'm wrong about the problem please try to explain it so I can understand better.

    Any separation between the blocks or plates and the rim should be aligned, closed and glued. That includes the hear block and the separation at the tail end. If you plan to do it yourself and don't have some sort of jig to align the rim with the top or back you'll need to make something.

    I suspect the "translucent layer" between the top and head block is hide glue, and it shouldn't be there at any thickness over a few thousandths. There was probably a gap there when Gibson glued the top on and it was filled with glue. If you simply glue it back together it will be roughly equal construction quality as the original, but if you want to improve it, removing all of the old glue and fitting a wood shim would be much better. (That assumes that the instrument has not been taken apart and re-glued before, in which case the layer could be some other glue or who-know-what.)

    Now that I've rambled on about that stuff, some pictures would sure help us to understand what's going on.

  3. #3
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    Default Re: Body movement in a 1915 A.

    The pictures I should have included (sorry):
    Here I clamped a piece of carbon rod to the neck to show its displacement below the level of the top. Also visible is separation between neck block and top edge of instrument's side:
    Click image for larger version. 

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    Here it is with a .012" feeler gauge:
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    Side/neck block separation, other side:
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    Side/back separation. I'll have to make a caul or something and be forceful:
    Click image for larger version. 

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    Layer of translucent material (about 1/16"):
    Click image for larger version. 

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  4. #4
    Registered User sunburst's Avatar
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    Default Re: Body movement in a 1915 A.

    OK, that changes everything... or nearly everything.
    The fingerboard surface of the neck will need to be milled for a good glue joint to the fingerboard, so milling it level with the riser block will take care of .012". It will clean up the old glue at least for the most part.
    Looks like a lot of stuff has been don by someone, apparently ill-informed about how a trus rod works and how to install one. There could be any kind of glue in there, I'd bet Titebond or equivalent and it all needs to go.

    It looks like the riser has warped and lifted from the top. Good luck getting it to stay glued down if it is simply glued and clamped. Better would be to glue a thin well fit shim into the opening, or better yet, remove the riser, re-fit and re-glue.

    In the last picture it looks like the top and riser have shrunken away from the rim because the head block in in there keeping the rim from moving along with the top. Nothing much to do about that, it is probably secure and solid.

    The missing substance is binding material of some sort, usually cellulose. If you can find a matching piece of binding simply glue in a piece.

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