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I am building a 4 string bass from scratch, this is my fourth so far and most have turned out well.

With this one I have experienced an issue I have not seen and would like some advice.

Basically I decided to put the bass together prior to some final finishing, and put strings on at tension and left it overnight. When I got back to it the next day the headstock had been pulled down by the string tension and the back of the neck split.

Where did I go wrong? I was getting a bit cocky with a few successes under my belt and did a few things new this time. I installed graphite bars which I thought would be great, but didn't help where this broke. I had attempted to make the neck a bit on the thin side, did I go too far? I am wondering if the headstock plate I put on made the headplate weaker? Or was it the back of the neck near the nut was too thin?

What could I do differently?

As always appreciate any suggestions!

You just went too thin. Not enough meat where it was needed. It’s hard to see the insides very well in the picture, it it looks like the graphite rods are visible, which I think went too far into the weakest part of the neck - around the transition to the headstock. Don’t let it discourage you, we’ve all made mistakes. I sanded a neck too far and exposed CF rods once myself.

My guess is that the graphite bars were not well-adhered to the wood, so instead of carrying the load, it was just like having additional voids in the wood. Do some glue testing to ensure that you get a strong bond next time. Wash the graphite bars with soap and water before glue-up. An invisible film of oil can decimate the strength of an epoxy bond on non-porous surfaces.

my first thought when looking at the picture was in line with Dennis' comments. It looks to me like the Graphite stayed in place relative to the neck shaft but as the stress was applied, the wood broke as if it were thin and empty not a solid beam of wood and graphite.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Ouch, that sounds rough! Building basses is such a rewarding but finicky process. It sounds like a combination of factors might be at play. The thinner neck profile, graphite rods not positioned ideally, and possibly the headstock plate all conspiring against you. Sometimes, you just want to kick the buddy who told you to try that thin neck! Seriously though, maybe reinforce the neck with some additional wood on the back before re-shaping? Good luck on the repair!

A crack in the headstock sounds like a luthier's worst nightmare. Graphite bars, eh? Maybe they shifted the stress. That thin neck might be the culprit too. Did that headstock plate add structural weakness, or was it the nut area? It's a real head-scratcher. I remember one time, fixing a vintage guitar, thought I'd nailed the neck angle, then bam the bridge popped off. Talk about frustration! Used too much glue apparently, not unlike our bass builder's issue, huh, where a love tester instrument could identify the breaking point.