1919 Gibson A-4 Mandolin: New Bridge & Crack Repair, Pt. 1

You may recall my Gibson A-4 mandolin from work I did on it a few years back.

The action on it is a bit high. I've lived with it for a while, but I decided to bite the bullet (or the ebony...) and make a new bridge.

A-style Gibsons of this era have a fixed, non-adjustable bridge, unlike F-style instruments, which have an adjustable bridge. On a more modern instrument, I would simply shave some height off the bridge to lower the action, but I don't have the heart to take a file to the original bridge. So I'll carve a new one instead.

I also have some new cracks to fix. I just procured a really cool, clever crack-fix tool kit, so this is a great opportunity to try it out.

I see a lot of these mandolins with cracks up near the fingerboard. Mine had one on the other side which I repaired, and now I need to fix this one.

The Stew-Mac crack tool kit consists of a plug cutter to make cleats and an air gun to help inject glue into cracks, as well as some wire and tie blocks (more on that later).

Here I am making some cleats for the crack. I've done this by hand in the past - but the plug cutter lets us make nice looking cleats quickly and easily.

Here's the cutter with a few cleats. Since the top of the mandolin is spruce, I used spruce. They look great.

We need to make a caul to help put leverage on the crack to level it and make it tight.

I grabbed a piece of scrap pine for the caul.

I also laid out the cleats in the approximate location I want them under the crack, and marked the caul with the location of the holes in the cleats to line them up.

I sanded the bottom of the cleat to the same curve that the mandolin has on its body where the crack is. Wanted to make it fit as closely as possible so as to maximize the tension I'll put on the cleats.

Drill holes in the crack that will line up where we want the cleats - and where we made holes in the cleat.

Cut a length of spring steel wire for each of the cleats - about 12 inches (3 cm) long.

Thread the wire through the holes into the body and pull it out through the soundhole.

On the other end of the wires, thread a cleat on, then tie a brass tie block on.

Here's the air gun. There's a suction cup on the end. Connect it to a compressor running about 10 psi.


Spread some glue over the crack, and use the gun's air pressure to help force the glue into the crack. I've just spread the glue by hand in the past, but having it under some pressure really helps get it into the crack.

The glue has a working time of about 5 minutes, so I had everything ready to go. Five minutes seems like a long time, but it goes by quickly when you have a lot of steps in the process.

I'd prefer to use hide glue, but I used Titebond because of the longer working time. No way I could do all this in the 60 seconds hide glue would start to set in.

Here's he crack with the glue injected into it. Looks good.

Spread some glue onto the top of the cleats where they will contact the underside of the mandolin's top where the crack is. Be careful not to get glue onto the top of the mandolin.

I also put some waxed paper inside the body near the crack to keep glue from winding up where I didn't want it.

Put some waxed paper over the crack so glue won't adhere to the caul.

Then I slipped the wires through the holes drilled in the caul, and pulled the wires through, until the cleats seated up under the crack on the inside of the mandolin.

Now for the clever part.

Pull up the end of the wire outside the instrument, clip off any excess, and then wind it onto the tuner/tension tool from the kit.

Then wind the tuner to pull the wire upward, and thus, the cleat up tightly into contact with the underside of the body.

Did I say clever?

So we have one tuner for each cleat.

Couple of things: the tuners are slotted, so the wire, which is a bit stiff, is tricky to get into position on the tuners. I put my finger over the slot to hold the wire down as I wound up some tension.

You don't need a lot of tension. You'll feel when the cleat is pulled up tight.

Stew-Mac sells extra tuners - I bought a couple. I would be easy to make these in your shop - using the tuners of your choice (i.e. not slotted) mounted onto a block. I plan to do this.

But overall, it's a great kit and, again the concept is very clever. You need cleats to help hold some cracks together, and this helps to clamp them into place.

Here's a mirror shot of the cleats from underneath.

You think it's hard to get a mirror into a guitar body and take a picture, try a mandolin!

The cleats look great - totally professional. And the repair is very solid.

Next time - more fun with cleats! And some finish touchup.