Unlike neck re-sets this is not a repair that every guitar will need. Some can go their entire lives without needing a bridge replaced. A bridge can need replacement for many reasons. Left in a car the glue can soften and the tension of the strings can pull the bridge up. The piece of wood used for the bridge can have an undetected crack and split. In the case of some Gibsons from the 60’s, the plastic (yes I said plastic) bridge is bolted to the top and usually warps over time. I’ve never been shy about what I think of a high priced guitar with a dime store bridge. This goes for bridges with adjustable saddles. These have no place on any acoustic! Older guitars using hide glue can pull off because of fatigue. Hide glue, being organic, can deteriorate over the years. The bacteria present can break down the glue. Some repair people shave the bridge down instead of doing a neck reset and this can weaken the bridge as well. This type of “repair” has fallen out of favor for obvious reasons. I just worked on such a guitar.
Now first we must evaluate whether or not the bridge is salvageable. If it’s not then we must find a suitable replacement. If it’s a bridge that’s common, such as a Martin, I just use that but sometimes, in the case of the plastic Gibson, you will have to make another one out of rosewood or ebony. Make sure that the pin holes are the same spacing and the saddle slot is in the same area. Of course if the intonation was off on the original, now’s your chance to correct that. If I’m making a new bridge, I will make it about a 1/16″ bigger all around to cover any damage that has occurred or can take place taking the old one off. Martin even sells such bridges. I start by planing or sanding a blank to about 3/8″, cutting to shape with a bandsaw, then drilling the holes for the pins and finally cutting the slot for the saddle. This is accomplished using a simple jig with fence attached to the router.
If the top has a dome shape (my tops are a 25′ radius) I usually sand that in before I cut it to shape. That allows me to get rid of any rolled over edges from the sanding. I do this by making a positive radius sanding block from my bigger forms used to radius the braces.
Once the bridge is completed, I make a heavy cardboard template for the top of the guitar, so that only the bridge is showing. This is covered in aluminum foil to shield the guitar from the heat source needed to soften the glue holding the bridge. I then use a silicone heating blanket with a timer and temp controller to heat the top of the bridge. You can also use a heat lamp for this but I find the blanket does a quicker job of heating just the bridge area. The blankets can go to 500 degrees so set the heater at about 250 degrees. I use a block of wood clamped to the bridge to ensure good contact and I usually let it sit for about 10 to 15 minutes.
Once the heat is on I use several palette knives and a thinned putty knife to get under the bridge. If It’s not budging, I put it under heat again. I’m cautious about too much heat because I don’t want to break the joint combining the two top plates. I’m also very careful of how much pressure I use with the putty knife. I don’t want to take the top with the bridge, so slowly is the rule. The bridge usually has enough mass to keep the heat sufficient to separate the bridge from the top.
If you’re going to use the old bridge, make sure you clean up any residual glue and splinters from the top. You must then do the same with the top under where the bridge was located. I use a sharp chisel to scrape away the glue. This is also a good time to check the bridge plate underneath the top for any repair there as well. The ball ends of a steel string can be tough on wood. If it’s a mess you can simply glue a small piece of rosewood or maple on the bridge plate or spend $150.00 on a bridge plate repair tool from Stew-Mac. While expensive the repair is almost undetectable.
I like to get my clamps ready at this time. Deep throat “C” clamps work well provided they have a block of wood attached to the stationary part of the clamp to clear the “X” brace. You can also buy clamps specifically designed for the task. I’ve made a clamp that is similar to one that Charles Fox designed that consists of an aluminum plate with two threaded rods that go through the outside bridge pin holes and mate with a top piece of aluminum. Once bolted together, there is no clamp to get in the way for cleaning up the glue.
Make absolutely sure both the surface of the top and the bridge are completely free of glue. Fresh glue will not stick to dried glue! Put a generous amount of glue on the bridge and don’t worry about squeeze out at this point. If you’re not using the Fox system you can use plastic bridge pins to locate the bridge and make sure it doesn’t move while you’re clamping.
Now, you can clean up the glue before it dries. Don’t worry if you find that some glue has escaped your notice as it will come off a finished surface when dry by using a soft cloth dipped in hot water.
Two more things have to be done the next day after the glue is cured. You have to ream the bridge pin holes to the correct taper of your pins (usually 3 or 5 degrees, depending on the pin). This is accomplished with a pin reamer (yes, yet another specialized tool) but in a pinch I’ve used the tapered end of a file by sharpening the tang. Lastly, the bridge pin holes have to have a slot in them to allow for the strings to seat. Saws are available for this but I like to use a Dremel and three different sized cutters to make the slots. The slot must be rounded over in the direction of the saddle so that the string does not break at a sharp angle.
Now you’re ready to string up and play.