Trussrods: Doing The Job Right
A local bass player named Greg brought in his beautiful Tobias 5-string for adjustment and setup. The action was impossibly high for him to play—about .15" at the 12th fret, and slightly more in the 7th- to 9th-fret area. The neck had a major up-bow (i.e., the fretboard bowed up from the pull of the strings). The two trussrods were fairly tight, so to get the neck straight, the rods would need to be tightened even more.
The correct approach to straightening a neck—as I’m constantly preaching—is to go through the following simple maneuvers first: (1) Remove the trussrod nut. (2) Clean and lube the threads; I use petroleum jelly. (3) Re-install the trussrod nut until its threads have just engaged the trussrod’s threads; then stop and back off the nut slightly so that the nut is loose. (4) Put spacer blocks on the fingerboard—one toward the nut end, the other toward the body end—and place a long, flat object on the blocks. (5) With a protective curved caul on the back of the neck, use a bar clamp to gently force the neck straight (see Fig. 1); continue with gentle clamp pressure until the neck is back-bowed a fair amount. (6) Tighten the trussrod nut until firm. (7) Remove the clamp and blocks, re-string, and check the fingerboard for levelness end-to-end.
I call this procedure “helping the trussrod do its work”—trussrods aren’t really meant to force a neck straight without this help. You may have to repeat the above maneuver several times if the neck doesn’t straighten on the first try. This neck adjusted perfectly, but I needed to make some decisions to fix minor problems along the way.
Since I didn’t have a custom wrench supplied by the manufacturer, the two trussrod nuts in Greg’s Tobias were not very approachable. Due to the trussrod-adjustment cavity’s shape, a volute of wood—which the trussrod cover screws into—blocked the wrenches I wanted to use. My guess is that the neck was last adjusted by a novice, and with the wrong tool. Imagine trying to straighten a big neck using a small open-end wrench (Fig. 2). I decided to eliminate the wood that was in the way in order to make room for my trussrod tool.
Fig. 3 Both of the q" hexagonal brass trussrod nuts were marred and misshapen from someone fumbling loosely on them with the wrong tool. One nut in particular was dinged up enough that my wrench had trouble slipping over it. So, I rubbed each of the hex nut’s flats on 320-grit sandpaper taped to a hard, flat surface until the wrench fit.
Fig. 4 Even with the wood volute removed from the rear cavity wall, I couldn’t quite make the sharp turn that would allow the socket wrench to slip over the hex nut. So, I ground off the face of the wrench to shorten it.
Fig. 5 I also ground down the rear side to shorten it further and to round it, too. The wrench now looks sort of like Popeye’s pipe, don’t you think?
Fig. 6 A good q" of wrench then fit snugly around the hex nut, covering the area I had marked with a blue felt-tip marker.
Fig. 7 Once the neck had straightened, I drilled a small hole in the cavity bottom for a longer screw to hold down the trussrod cover. I measured the body’s thickness and the depth of the cavity to ensure that I wouldn’t drill too deep and out the backside of this beautiful piece of work.
With the neck straight, I went on to other tasks: replacing several frets in the 1st position; leveling, crowning, and polishing the frets; installing a new nut (the old nut slots were far too low to work with a straight, low-action neck); and setting up the bridge-saddle radius to match the fingerboard’s.
I’m always amazed at how much can said about a “simple” task such as adjusting a trussrod!
