The Case Against Bolted Bridges

Late '60s Kay with a shaved-down bridge and decorative brass bolts

Bridges attached via nuts and bolts can be found on a variety of instruments from the most basic Harmony to pricier Gibsons. They're something that came about as a way to help hold instruments together but I consider it to be bad practice and dangerous.

Spruce has a Janka hardness of around 400 lbf (pound-force) compared to East Indian Rosewood's 2500 lbf and Hard Maple's 1500 lbf. Acoustic string tension puts about 180 lbf of force onto the soundboard that wants to pull and twist the top up towards the neck. Mathematically that seems like it should be fine but there is more at play than just a wood's resistance to denting. On a typical acoustic guitar, the ball ends of the strings chip away at the Maple bridge plate over years of string changes. This allows the force of the strings to be focused on a smaller area as the ball end begins chipping its way out. This leads to such problems as a bellied top, a lifting bridge, or the windings of the strings starting to creep closer to the saddle and tear things up.

Repairs can range from capping the damaged portion of the bridge plate to a Bridge Doctor to pull the top down to a full on bridge refitting and reglue. Capping the bridge plate with Maple and redrilling the holes is one of the more common repairs I do on old acoustic instruments and especially ones with Spruce bridge plates (a terrible idea).

They'll never separate

From an engineering perspective, I can see the appeal and the logic behind them and how it would make sense from a carpenter's point of view. When you want to keep two objects bound together you can choose to glue them or bolt them together. Doing both allows the bolts to keep the pieces together if the glue begins to fail and so, in theory, it saves the guitar from having to undergo an expensive bridge regluing

It often comes up in less expensive guitars like Harmony and Kay guitars because it was meant as a reinforcement for the poorly prepped gluing surfaces for their bridges. Often the lacquer underneath the bridge was lightly scored and the bridge was glued directly to the the finish which prevents true wood-to-wood contact. The bond between lacquer and wood is so much weaker than the bond between glue and wood and so each of these bridges are destined to fail. The inclusion of bolts in the bridge was meant to buy these guitars more time before the effects of shoddy construction caught up with them.

1940's Kay acoustic with a poorly prepped bridge area and visible bolt-holes. 

The instrument pictured above came to me without its bridge and based off the oxidation it seems like it has been gone for a very long time. Luckily someone noticed the bulging top (due to the Spruce bridge plate) and managed to fairly cleanly remove the original and install a cheap floating replacement. That, although not ideal, is the instrument's saving grace as prepping the area for a new bridge is much earlier than patching missing wood if the bridge had been allowed to tear out.

We want bridges to fail

Hear me out on this, it is better for an acoustic bridge to pop off than to rip off

The 1960's Kay with a properly prepped bridge area

Bolts do not solve the problem of a wandering bridge but are rather indicative of a "band-aid" fix that will cause more damage in the long run. When the glue joint fails and the string tension is countered solely by two bolts and nuts (usually with small 1/4" washers) then the result is that the wood will begin to flex until the nuts are torn through the top. The aftermath is that the bolt and nut are still attached, being the strongest bond, and you now have a nasty, splintery mess where your bridge used to be. 

When should I remove the bolts?

If you've got a vintage instrument with significant value (>$1000) or originality then I would leave them alone for the sake of not messing with the value. The vintage market is weird and sometimes people want those things kept on.

I typically opt to remove them as I work on less expensive vintage instruments and I'll usually end up doing a proper bridge reglue. If the instrument has had a proper gluing and fitting then it is highly unlikely that the tension will lie solely on the bolts to cause such a problem but I still maintain that it is better to remove them.You never know when your instrument might get hot enough or wet enough to revive the glue and then your simple bridge reglue and warning from your luthier turns into a mess of patching and gluing. I always keep the pearl dots because in a lot of ways its thought of more as a cosmetic choice than as a disguise for what is underneath them.

There might be something to be said about removing that much dead weight from the top but I cannot back that up with any evidence.