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Straightening a twisted neck with a backbow

Discussion in 'Hardware, Setup & Repair [BG]' started by GlennW, Aug 11, 2007.


  1. GlennW

    GlennW

    Sep 6, 2006
    I'm writting this because there's next to no info about this stuff online and it might help someone. I just got a Harmony H22 project bass, just the neck, body, and three screws that hold them together. Upon arrival the neck had a bad backbow, truss rod adjusting nut that wouldn't move, and what appeared to be a counterclockwise twist when viewed from the peghead end.

    Some carving with a small screwdriver in the truss rod nut channel made it so a 1/4" box end wrench would fit. Before moving on to the neck I took some measurements with the neck relaxed (truss rod nut removed).

    E-side: (all measurements are approximate)
    Straightedge would rock on the 6th fret
    With 0" clearance at 1 and 6, there was about .076" at 20
    With 0" clearance at 20 and 6, there was about .048" at 1

    G-side:
    Straightedge would rock on frets 7 and 11
    0" clearance at 1 and 7, .051" at 20
    0" clearance at 20 and 11, .071" at 1

    Before jigging I was under the impression that the G-side was backbowed more than the E-side as a result of the neck twisting. I got an idea - to set it up with a significant bow, but to only heat the G-side. I figured (hoped) that after it cooled and the clamp was removed that the E-side would stay about the same since it wasn't heated, and the G-des would straighten out some removing some of the twist. It was jigged up with an .080" bow on the G-side (couldn't measure E-side, clamp was in the way) and two 250W reflector infrared lights pointed at the G-side.

    After about 3 hours I figured that this wasn't such a good idea. That's one good thing about the heat lamps - they give you plenty of time to think and regroup. I thought it would be safer to try to correct the backbow first, and then deal with the twist. Otherwise it could easily get worse. I put one light on each side, and later on tightened the clamp for a .100" bow and left it that way for several hours.

    The clamp was removed many hours after it cooled. Using the straightedge it looked like the G-side was almost straight and that the E-side had a ski jump around the 11th or 12th fret. At this point I had a revelation and concluded that my original diagnosis was wrong - now it was looking like instead of the G-side being backbowed more and twisting down, the G-side was closer to normal and the E-side was twisted high (because of the ski jump). I was glad I rearranged the lights.

    Round 2. It was jigged with a backbow, clearance was 0" at 20 and 9, .085" at 1. One light on the E-side, no light on G-side. After a couple of hours I thought this was bad because the straightedge was only rocking on 1 fret and it might end up adding a backbow to the ski jump and make a mess. The clamp was loosened so the neck was more arched (straightedge rolled across two or three frets instead of rocking on one) and the clearance was about .025-.030".

    I left it like that and tried to imagine how it would turn out after cooling and how to jig it next. I was surprised to find it in what might be playable condition. It looks very out of whack overall, but the straightedge shows it being pretty good as far as the individual strings. I don't know what's going to happen once there's string tension, or how it will react to truss rod adjustments - it might want to kink sideways, it might act nice. If you're in a chair and set the heel on the floor it looks like the nut end of the fretboard is twisted a little counterclockwise (as before) and the headstock is twisted clockwise. Strange.

    It needs the holes for the tuning keys doweled and re-drilled. I'll borrow some TI flats from another bass and try them on this since they're low tension. I have a feeling that stiff strings might might be a good idea on this one.
     
  2. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Glenn, I strongly suggest that you do a search on the use of 'winding sticks' as a means of determining exactly where, and how severe the neck is twisted.

    You are lucky that the TR nut is not twisted off. The truss rod in your Harmony is not a compression rod like in a Fender. It's a dual rod, bending type rod. I'm attaching a pix of one that was broken and repaired in my H-27.

    If it's not already too late, I suggest that you remove the position markers from the FB for reuse. If the neck is made hot enough to straighten the neck, it will ruin the inlayed dots.

    Does the H22 have a bound fingerboard?. If so, be careful not to get the neck hot enough to lift the binding.

    I think that, were I trying to do what you are trying to do, I would remove the FB and plane the neck flat.

    Another bit of advice: Inspect the bridge area of the top plate and make sure that the top is not deformed under the bridge. If it is substantially deformed, it's hard to fix, and the geometry of bass will be compromised making a good setup impossible.

    By the way, I know you posted a pix earlier but I can't seem to find it now. Where is the pix?
     

    Attached Files:

  3. GlennW

    GlennW

    Sep 6, 2006
    Thanks for the pic, I'll research winding sticks.

    Here's a H22 page.
    http://harmony.demont.net/model.php?id=110

    It doesn't have a bound fingerboard, and the dots survived. The lights were aimed directly from the sides of the neck, with the top half of what looks (and feels) the hottest circle of light on the neck.

    Here are a couple of shots I just took. I laid a straightedge across the body and it seems ok - these are flat top, not archtops like the H27.
     
  4. GlennW

    GlennW

    Sep 6, 2006
    It's alive.
     
  5. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Looking good!!!
     
  6. juniormarbles

    juniormarbles

    Apr 19, 2008
    ...to adjust my rod's nut (what size is it?)

    I wondered whether the re-issue H22 may possibly use the same nut, so I may obtain such a wrench from the importer?

    I read this thread, but I am not into carving any more wood out of an already pretty thin juncture, to make room for a regular-sized nut driver. I' d really like to adjust it with a very thin-walled, properly sized wrench, as it came supplied when the bass was originally sold.

    Any help would be appreciated.

    Best regards,
    JM
    (juniormarbles@gmail.com)
     

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