Repair of a 1931 Martin OM-45

[pheumiller]

Folks, we just posted a blog chronicling the repair of a rare 1931 Martin OM-45. Following is a quick snippet of the article. You can then click the link below to keep on reading. Hope you enjoy:

With vintage instruments, there’s simply no telling what patches and pokery you’re going to find under the hood. Good news! Dream Guitars is well-acquainted with those surprises, and we know just what to do when they crop up. Recently, a client and collector came to DG with several obscenely rare Martins, among them a 1930 OM-45 that he was concerned was too quiet. Once Paul Heumiller got his hands on the guitar, his ear told him something was definitely awry. Having played many of the “Holy Grail” Martins for the 20s and 30s, he’d expected to hear a energetic voice with vigorous projection, but this guitar sounded timid, with a bad case of congestion.

Paul immediately suspected that the bridge plate had been modified, and after plumbing the depths of the OM-45 with a flashlight and a mirror, his suspicions were confirmed: glued where the original bridge plate should have been was a massive (over 3″ wide) modified bridge plate! It was probably installed in an effort to combat the tendency of the top to belly up as the string forces enact continual upward stress for years, over-doming the top around the bridge. On the one hand, the girthy bridge plate worked perfectly: this more-than-80-year-old guitar had very little belly to its top, unheard of at its age. On the other hand, the voice was something between a Chevrolet sputtering tailpipe fumes and a cat mewling in the rain, the top’s vibrations were so severely dampened

Click here to keep reading...

[mirwa]

Nice old guitar, I had a read of your blog, stuck as to why it required a hole to be cut in the top where the bridge was

Steve

[LouieAtienza]

Originally Posted by mirwa

Nice old guitar, I had a read of your blog, stuck as to why it required a hole to be cut in the top where the bridge was

Steve

I believe the hole was from the original repair, not the current restoration. Possible the strings pulled through the top originally, and that hole patch and bridge plate were there to "fix" it...

Awesome project Paul, and hope to see the rest of the restoration work!

[pheumiller]

That was an existing patch in the top around the bridge pin holes. It probably would've come off with the bridge plate anyway, so we took it out, which also gave us convenient access to the interior of the bridge plate.

[mirwa]

They are the worst things to have to fix, that is other peoples bizzare ideas of repairs.

Glad to hear you did not put the original hole in the top

Steve

[redir]

I've done a repair just like that patched up job in the past. When a bridge pulls up and takes half the wood with it and leaves a hole right through the top then you patch it up and then make a bridge plate to cover it. The bridge plate they covered it was excessive though.

John Arnold, Willi Henkes, and Dennis Berck have all demonstrated similar methods for repairing such damage under the bridge with an overlapping spruce patch and regular sized bridge plate. This is a critical area of the guitar's structure and some repair methods are just tone killers. Their results border on miraculous.

[mirwa]

Originally Posted by redir

I've done a repair just like that patched up job in the past. When a bridge pulls up and takes half the wood with it and leaves a hole right through the top then you patch it up and then make a bridge plate to cover it. The bridge plate they covered it was excessive though.

The issue I have with repairs as shown by that photo, is you are cutting a hole into the top, and leaving end grain exposed as the point of contact for a repair.

Then inserting a piece of wood, that again is cut and square with just end grain to join,

So that's two end grains being put together and expected to hold, these two end grains are then accompanied by another section of two end grains on the opposite side of the insert.

Makes no sense and provides no strength to an area that desperately needs the strength.

To repair a previous repair like this, I like to get rid of the end grain situation and actually extend the repair out further and have it tapered so your insert gets a surface to surface join. Sometimes that means you have to do some cosmetic work on the exposed top, but it's worth it.

Steve

[redir]

Yeah I think we are talking about the same thing. It's definitely not an ideal situation. I would also extend the areas of the bridge and the patch so that it 'clamps' the whole thing together.

Originally Posted by mirwa

...To repair a previous repair like this, I like to get rid of the end grain situation and actually extend the repair out further and have it tapered so your insert gets a surface to surface join. Sometimes that means you have to do some cosmetic work on the exposed top, but it's worth it...

Using the method I noted above there will not be any work outside the footprint of the bridge, unless of course there was already damage there. All the overlap is inside the guitar. This is especially important in maintaining value for vintage instruments.

[John Arnold]

I have been doing interior spruce patches under the bridge for almost 25 years, and I have saved some tops that other repairmen certainly would have replaced.

http://theunofficialmartinguitarforu...2#.V1nfHNQrL6o

[Bruce Sexauer]

There are two ways to replace the "bad" spruce that I would have preferred. Here is a sketch which should be self explanatory:



I would expect HHG to have been likely able to glue a bridge down even w/o removing the epoxy IF the surfaces could have been mated, as appears possible to me.

[pheumiller]

The last time we checked in, this 86-year-old Martin had been divested of a poorly-executed repair that was compromising its top, with the last of the lingering epoxy scraped clear and the massive bridge plate and patch removed.

Much has happened since then! After brainstorming and consulting with other maestros of the repair world, Ken Jones and our team have landed on an elegant solution to simultaneously reinforce the top, fill the rectangular hole between the bridge and the bridge plate, and create a consistent platform to reglue the original ebony belly bridge: combine the patch under the top with the plug into one patch-plug.


Spruce stock has been carefully trimmed to exactly fit inside the braces, then excess spruce has been routed away from the plug portion. The thickness of the top of the OM-45 is about .112″, so this piece was thicknessed to .168″ or 1.5x the thickness of the top.

Next, the thickness of the top (.112″) was routed away everywhere on the patch, except for where the patch will fill the hole in the top.

Before we could even consider starting on this exciting new leg of the repair, we first needed to patch the sections of the X braces where the previous colossal bridge plate was notched into them. If we hadn’t addressed those gaps, the top would have been at significant risk for further deformation. That finished, we then moved on to the spruce blank itself, shaped to fit between the newly-repaired X braces. The top of this Martin OM-45 is .112″, so the spruce blank* was sanded to .168″, or one and a half the thickness of the top, so that, once installed, the plug would sit slightly proud of the surrounding top. After transferring the shape of the rectangular hole in the top between the bridge and bridge plate to the blank, we then carefully routed the surrounding material until the “plug” part of the patch was .168″, and the surrounding spruce was .056.

Then, the location of the bridge plate was marked out on the underside of the patch, and the patch was then sanded from that point of contact to paper-thinness at its edge to minimize mass and allow the original top to vibrate as freely as possible without jeopardizing its structural integrity.


An edgewise view of the tapered back end of the spruce patch.

After looking at these photos, you might be asking yourself, “Why is that thing so wide?”*The patch-plug has a large surface area in order to increase the amount of gluing area for the patch-plug, meaning that the patch-plug is glued to the underside of the top in addition to being glued to the end-grain of the sides of the rectangular hole left by the previous repair. By itself, an end-grain glue joint is inherently weak. However, with the additional gluing force of the lower section of the patch against the underside of the spruce top, we’re confident that this new patch-plug won’t pull up, which was the problem with the epoxied patch from an earlier repair which we had chiseled out. The large surface area of the patch-plug also provides a little more reinforcement to combat the bellying that was present when the OM-45 first appeared on our bench.

The patch-plug was then carefully scored partway along spring growth grain lines (which are softer) and broken into three pieces that equally divided the plug in order to fit into the soundhole, with blue tape on the back to act as hinges.


The “top” or the non-show side is scored about halfway through the patch with a sharp blade The score lines are in the soft, spring growth grain. Next, it is broken along those score lines. This is to prevent a seam from showing inside the guitar once it is in place. Also, a break will help register all three pieces together cleanly inside.

The moment of truth came when we first folded the patch-plug and eased it into the soundhole before*pressing it into place between the X braces. Turns out: a perfect fit.*


So snug, it will stay in place on its own without glue or tape!

Once we knew the dimensions of the patch-plug were exactly what we wanted, we then made several cauls for the top and bottom, and aged the new spruce patch-plug with Potassium Permanganate in order to make it blend in better with the surrounding 86-year-old spruce.


A special set of inside clamping cauls are mad to fit the patch.

Now, a test fit is all fine and dandy, but it’s another matter entirely when the moment of the actual glue-up arrives. This time, everything must to be perfect. In keeping with tradition, we used hide glue and several cam clamps and deep-throated C clamps with the cork-lined cauls to cement the patch-plug with the original top. Hot hide glue is absolutely essential for a repair of this nature, where even clamping pressure is key, and hide glue’s ability to pull the wood tighter and tighter together as it cures helps ensure that the joint is airtight and even. Moreover, hide glue is tonally superior to rubbery aliphatic resin-based glues–but this comes at a price: a very short working time. Thus, appreciate how cool-headed and savvy is our repair staff:*we were able to evenly spread glue on the patch, fold the patch, get it inside the guitar through the soundhole, unfold it, put it into place, then precisely arrange three cauls and five clamps inside a tiny soundhole, working by feel, before the hide glue could gel!


Only hot hide glue will do for a repair like this. Even clamping pressure is key.

From the initial dry fit to the actual glue-up, this patch-plug was a star patient, and we’re quite happy to report that everything fits snugly, and the plug sits just slightly proud of the surrounding spruce.


The plug ended up just proud of the top so it can be sanded down to level.

Next on the docket? Sanding the plug flush to the top, then levelling the entire surface under the bridge. This last step is crucial and delicate: without a consistent gluing surface to attach to, the bridge will invariably pull up again.

As the work progresses and we get closer to refitting the bridge and restringing this Holy Grail Martin to its former glory, we’ll post more photos with each swipe of the sandpaper. We’re getting very close to hearing what it sounds like to right the wrong of that gruesome earlier repair!

[mirwa]

I had written a response, have thought about how it may come across, so have since deleted it.

Steve

[LouieAtienza]

Thanks for sharing Paul, can't wait to see the final installment....