Acoustic Guitar Bridges

Gluing A Loose Guitar Bridge

It is important to check the glue joint between the bridge and top to ensure there are no gaps.


Thin paper slid beneath loose bridge.

A thin piece of paper can be used to determine the extent to which the bridge is loose. If there is evidence that the bridge is loose it should be inspected and repaired if necessary.

Neglecting a loose bridge can create more damage if it breaks free and tears fibers from the top.

A loose bridge can also shift forward and damage the finish.

Possible Causes:

Before a loose bridge is reglued it is removed from the top. I use specially designed heating blankets which direct the heat to the bridge and avoid overheating the top or finish. A flexible, smooth spatula serves to separate the bridge from the top once the adhesive is softened.

c clamp and padded caul clamp bridge to top for gluing
Regluing loose bridge

All old glue is removed from the top and bridge to provide a clean surface for re-gluing.

A bridge caul is used to evenly distribute the pressure and a clamp is used with exterior and interior cauls.

Heat / Dryness Can Soften Glue

Leaving an instrument in a hot car, attic or other hot environment may cause the glue to soften which can allow the bridge to shift or creep forward. Unfortunately this usually does some damage to the finish as well.

Loose X-Brace / Top Deformity

The X-brace helps to stiffen and strengthen the top of the guitar. It runs beneath the ends of the bridge and helps prevent a certain amount of top "rotation" and arching. When braces come loose string tension can change the shape of the top, causing excessive bellying behind the bridge, unsymmetrical bumps in the top and even a concave dip in front of the bridge.

If the top's shape changes radically it can place incredible pressure on the glue joint between the top and bridge. Structural repairs are made prior to regluing the loose bridge and sanding of the bridge's base can improve the fit.

Bridge Glued To A Painted Surface

I don't believe I have ever seen this on an expensive instrument. Obviously this undesirable technique is chosen because it saves the factory time, however, it usually has poor results and almost always means the bridge will come loose ...eventually.

glue sits on top of finish of failed bridge joint
Finish was not removed prior to bridge
gluing.

The finish must be removed so the bridge can be glued to the bare wood of the top.

If the finish is very thick this can create a cosmetic problem. See Special Considerations below.

Bridge Plate

A cracked or warped bridge plate can also allow the top to deform, placing incredible tension on the top to bridge glue joint.

Insufficient Glue

Excessive clamping pressure may force too much glue out of the glue joint, though this is rare. Usually I see this on instruments where the bridge was not glued with traditional wood glue.

Other Common Bridge Problems

Cracks

Cracks normally develop in two places on acoustic guitar bridges; thru the bridge pin holes and at the edges of the saddle slot. Minor cracking thru the bridge pin holes can sometimes be repaired when there are no other issues with the bridge. It is usually inadvisable to repair cracks at the edges of the saddle due to the constant tension.

Warping

Bridges that have been loose for a length of time can distort in shape. If enough twisting occurs replacement is necessary.

A small amount of warp may be repairable by heating and clamping the bridge or planeing of the bottom when the bridge is thick enough.

Thinned / Modified

A neck reset is a costly repair to consider on inexpensive instruments. In an attempt to lower action on an instrument that needs a neck reset the bridge is sometimes thinned or modified. This is done because the saddle is already as low as possible.

When resetting the neck on an acoustic guitar who's bridge has been previously thinned it is advisable to replace the bridge to return it to the correct height.

Incorrect Saddle Placement

Intonation problems caused by an inaccurate placement of the bridge and/or it's saddle may also require bridge replacement. My article on intonation will shed more light on this subject.

Does the bridge effect tone? The bridge is a top brace, albeit an exterior one. A loose bridge weakens the structural integrity of the top and can affect the tone as a result of the poor coupling.
An overly thin bridge also changes the dynamics of tension/torque placed on the top and the distance of the strings from the soundboard. 

Special Considerations

Painted Guitar Bridges

Removing certain painted guitar bridges poses another problem. Most manufactured classical guitar bridges are glued to the top before finish is applied. When these must be heated for removal the finish would need to be stripped to avoid a mess. Any plastic inlays on the tie block would also suffer damage. This requires stripping and refinishing of the bridge. For this reason it is often easier and cheaper to replace these when necessary and available.

Plastic Inlays

Some acoustic guitar bridges have been inlayed with plastic (celluloid) inlays. The heat necessary to soften the glue and remove the bridge will normally destroy celluloid, it must be removed or replaced. Heat does not harm actual pearl inlay, it simply melts the glue.

Thick Top Finishes / Poor Surface Preparation

In order to reduce prep time some factories may forego removing the finish beneath the bridge. Bridges glued to finish often pop loose, sometimes leaving pieces of the finish still glued to the bridge.

Other manufacturers leave a small outline of finish around the bridge to ensure no bare wood is exposed around the edge of the bridge. It's important to realize that not all gaps around the edge indicate a loose bridge, as mentioned before, using a thin piece of paper around the edge can help determine how loose the bridge is.

The Exception

The only time that I would avoid removing the finish and instead, glue a bridge back down to it is when the finish is terribly thick. Think import guitar with polyester finish!

A very thick finish is not only difficult to remove, it leaves a ledge of considerable thickness which would be hard to disguise. While this means the bridge may very well come loose in the future, the alternative is unsightly. One would need to scribe around the bridge, clear the finish and "inlay" the bridge into the clearing.

See Also: Replacing Acoustic Guitar Bridges

Repair Tools

Bridge Gluing Jig

Bridge Gluing Clamps

Water Soluble Wood Glue

Hide Glue

Bridge Gluing Jig

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