Neck problem (Acoustic bass guitar)

[Brad Elbein]

So I dry-fit the neck to the acoustic bass guitar today, after 51 weeks of not touching the thing. Some questions arose ... note that this is a kit build with a bolt-on neck, two bolts, pre-drilled, so que esta, estara siempre. In terms of most important to least important problems:

TWIST: I looked at the neck to see if it was straight, and discovered a twist. It's pretty minor--a degree or so. Remember, this is a 19 1/2" neck (neck length, not scale length!) so it's not much of a twist over that stretch. But a twist nevertheless. Here's my sketch: [ATTACH=CONFIG]126208[/ATTACH]

Damn, it wouldn't insert.  What it shows is that at the point where the neck meets the body, with the horizontal plane of the body as a reference at 0 degrees ... at the fret immediately adjacent to the body, there's a twist from bass to treble of -0.5 degrees; at about the 12th fret, a twist of about -0.8 degrees; and at about the second or third fret, a twist of about -1.0 degrees.  (Which is all to say that if you're standing at the butt end of the instrument laying on the workbench, looking along the body and up the neck, there's a twist from the treble side of the neck toward the bass side.)   
Two questions: Is this small a variance a problem, and what do I do about it?
 

SPACE: When I laid the fretboard on top of the neck, with the neck bolted in place, there is a 3.75 mm gap between the body-end of the top of the neck and the top of the body, measured at the bottom (body end) of the neck. Of course it's a shallow triangular gap, decreasing to 0 as it approaches the area under the first fret. I can't move the neck up toward the top of the body to get the body and neck flush b/c of the pre-drilled bolts. So ... what? It seems like my two choices are (1) trim the top so that the fretboard lies flush (ugh) or (2) add some sort of "ramp" to accommodate that gap. I suppose there's (3) throw the damn thing out and start over, but we've been through so much together, there must be another solution.

 ANGLE: The angle of the neck, as near as I can figure, is 0.8 degrees relative to the top. It's designed for 0.5 degrees. Is that enough of a problem that I should start adjusting it, or is it a "leave well enough alone" kind of thing?

Thanks in advance for all of your (usually) excellent advice.

[Ned Knepp]

Hi Brad.

 I'm not anything like an expert or even a professional so take that into consider as you read this, OK.

 If it were mine, I'd plane the neck to make it flat and straight, removing as little material as I could to get it that way. I'm assuming that there is a truss rod installed too so it's fit may need to be adjusted too before you move on. 

The space you have between the fingerboard/top of the body and the top surface of the neck needs to be adjusted rather than filled. If the holes in the neck block are in the wrong place you can do a couple of things. The first/easiest is to elongate the holes vertically so there is a BIT of room for movement. IMO, the other choice is a better fix but means that the existing holes need to be plugged and new holes in the proper position need to be drilled. 

The neck angle depends on a number of things such as the bridge height, string gauge, playing style, neck relief and other things. Before you do ANYTHING with that you need to get the alignment issues worked out.  If you haven't already done so, you should spend some time reading about the final setup so you understand the relationship of all the things involved. It will help you understand the neck angle as well. I'm not trying to make it sound hard but  there are some "gotcha" if you just address one thing at a time without considering the others and you may end up with a less than desirable setup because of a small mistake made early on. 

The best advice I can give you is to take it slow and make sure you know what you are doing before you starting cutting/drilling/sanding/ gluing. 

[Mark McLean]

Brad

I agree with Ned.  The holes in the neck block are too low and you need to fix that.  Sounds like there may have been a miscalculation at the time of fitting the neck block and you didn't account for the thickness of the soundboard.  It can probably be remedied as Ned says by altering the holes, or filling and redrilling.  Then the neck will sit higher.  You can adjust the angle later, once the position is OK.  If you have already cut the heel to length it is going to be short now, but you can add a bigger heel cap to fix that.  It sounds like it is all fixable. 

Mark