The side splint
Reinforcing Side Cracks
© Frank Ford, 2003; Photos by FF

A blow or pressure from the outside is the most common cause for cracks in the thin sides of acoustic guitars. Once cracked, the side is a bit more vulernable to breaking again, particularly in areas where the side is relatively straight and unsupported. A the upper and lower bouts, the side is usually bent into a strong arched shape that's not nearly so easily bent by mild pressure from the outside.

By contrast, the straight section of a dreadnought's side lays right on the player's thigh, and is very easily bent inward by the downward pressure of simply playing the guitar normally. If there's a nice big bunch of keys in the player's pocket, the inward bend of the side is often enough to produce a serious crack. I believe such cracks need to be well reinforced inside, to resist breaking a second time under normal use. After aligning, clamping and gluing the crack, I'll set about reinforcing the inside.

My favorite reinforcement is one or more "splints" that span the entire side between the linings. I make the splint of mahogany, about 3/32" thick and 5/8" wide:

The beveled edges make the splint look less bulky, and the beveled ends flex just a little under load so the splint won't tend to pop off if the side gets leaned on from the outside. Even if the side crack runs right a the edge of the lining, this splint will keep it tight without having to bridge over onto the lining. The idea is to keep the side from bending inward under load, which, of course, was the way it broke in the first place.

I have a pair of acrylic cauls into which I've inlaid two 1/2" diameter x 1/4" thick rare earth (neodymium) magnets. These magnets are super strong, and with two in each block, they have a powerful attraction. Because they are mounted with alternate poles facing upward, the blocks are also self-aligning:

I usually use these smallish ones, but I do have a much stronger set for flattening the side as I clamp. It has 5/8" x 5/8" magnets and a very strong pull. More on that big set.

Outside the guitar I locate the side reinforcement exactly where I think it would do the most good, and lay one of the magnetic cauls right next to it:

Then I stick the matching magnet on the inside, so both are centered and aligned right next to where the reinforcement will go:

Now I have a perfect place to stick the reinforcement patch, so I can work entirely by feel:

What you can't see in the above photo is me placing the glue coated patch right up against the inside block. It ends up looking like this:

To the right of my caul, you can see an original side reinforcement. It's a bit thinner than mine, and it wasn't stiff enough or located in just the right place to prevent the side crack.

I typically use hot hide glue for these patches because it holds like the devil, has a quick grab as it sets so it won't slide around as I work, and it works well with relatively light clamping pressure.

Once the splint is in position, I lift the magnet off the outside, place the inside magnet on the splint, and stick the outside magnet back on, effectively clamping the splint in place:

My long side splint ends up looking pretty much like original bracing, and it does the job of holding the side together better that a string of little cleats because it resists the inward bend of the side when it gets leaned on or bumped:

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