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From a pratfall. . .
Cracked End Block
© Frank Ford, 7/12/02; Photos by FF

This Martin D-28 took a hard hit on its butt, probably in a fall straight downward on the end pin.
The pin was driven into the end block, splitting it right in the middle along the grain. (This and the other interior photos were taken through a flat mirror laid inside the guitar, and because of the camera angle, there's a bit of distortion, but I think they came out well enough to show what's going on in there.)
I had to do some of the work without actually being able to touch the affected area, so I used a long rod with a palette knife blade soldered on the end as a probe.
First I spanned the entire guitar and clamped the crack inward, so it would open widely inside the guitar.
Then I was able to use my long handled tool to work some glue into the crack. In fact, the crack opened so wide I had no trouble filling it with glue. As you can see, I left the end pin in place, jamming it a bit for a little extra wedging action on the crack.
After I removed the clamp, the crack closed up, squeezing out a bit of the glue, so I mopped up with a piece of wet towel and a long hemostat.
I wanted to reinforce the inside so the crack would not be likely to recur if the guitar received a similar blow, so I made up a mahogany patch that pretty much covered the inside surface of the end block. .
Unlike so many other crack repairs, this one has an easy solution for clamping. I slid the patch onto a 1/4-20 threaded rod, followed by a heavy caul and a nut.
A lot of nice glue on the patch for a good grip on the end block I like Titebond for this particular job because it gives me plenty of "open" time before it needs to be clamped, and since it's not a stressed joint, the low heat resistance of Titebond is not an issue, even if the guitar gets left in a hot parked car.
Here's a little trick I've been using for a while now, and it seems to help. When I'm clamping a broad patch that is tapered quite thin at the edges, I'll wet the back side of the patch in that area to avoid having water from the glue cause it to curl away from the surface to which I'm clamping.

From the outside, I was able to draw the whole business up really tight against a thick acrylic caul so the end block would be flat and solid as it was originally. I added a clamp over the top and back of the guitar to further close the crack, but somehow forgot to take a picture of that one.

Before pulling the clamp up tight, I found I had a bit of trouble aligning the patch on the inside, so I added a clear acrylic caul that was about the same size and shape as the patch. That way, the assembly tended to guide itself into location.
Now, the end block is quite a bit stronger than before the damage. Not that I'm suggesting the guitar would or should take another fall!

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