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Patch repair of sound post crack


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The Weisshaar book suggests a patch size of about 4.5cm long by 2.5cm wide if "only one crack is present and no other repair considerations need to be taken into account." According to this, 1 1/4 inch is a little short. And it suggests hollowing "the critical area along the crack to a thickness of about .5mm."

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The Weisshaar book suggests a patch size of about 4.5cm long by 2.5cm wide if "only one crack is present and no other repair considerations need to be taken into account." According to this, 1 1/4 inch is a little short. And it suggests hollowing "the critical area along the crack to a thickness of about .5mm."

I use this method and it works fine only thing I can add to Brads info is make the patch oval shaped and bevel the edge of the hollowed out place somewhat like a spoon and the edge of the patch apply with the grain going opposite from the back grain.Use chalk to make sure the patch fits were the hollowed out place is to get a flush fit.

Monroe

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Is there a consensus of the patch thickness and size?

If the S P crack is long, saying from saddle to the upper eye, will 1 1/4" size patch plus cleats be sufficient for repair? Thanks. :)

I think 1 1/4" (3.2 cm) is a bit on the small side for a reinforcement patch over the post.... I'd opt for a slightly longer patch in most situations.

It's important that the patch bed extends most of the way through the plate (as Brad mentions, Weisshaar suggests leaving only about .5 mm of original material over the crack line) and that the bed walls are prepared carefully (curved with no "bumps"), or fitting the patch will be quite difficult.

I've installed both oval and elongated patches. Both work. I like the look of the elongated type a bit better, personally (photo below).

I believe Joe Grubaugh and Sigrun Seifert produced an article that appeared in Strad concerning fitting and registering patches (maybe a year or so ago?). Worth reading.

Cheers!

patch.jpg

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I think 1 1/4" (3.2 cm) is a bit on the small side for a reinforcement patch over the post.... I'd opt for a slightly longer patch in most situations.

It's important that the patch bed extends most of the way through the plate (as Brad mentions, Weisshaar suggests leaving only about .5 mm of original material over the crack line) and that the bed walls are prepared carefully (curved with no "bumps"), or fitting the patch will be quite difficult.

I've installed both oval and elongated patches. Both work. I like the look of the elongated type a bit better, personally (photo below).

I believe Joe Grubaugh and Sigrun Seifert produced an article that appeared in Strad concerning fitting and registering patches (maybe a year or so ago?). Worth reading.

Cheers!

patch.jpg

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I use this method and it works fine only thing I can add to Brads info is make the patch oval shaped and bevel the edge of the hollowed out place somewhat like a spoon and the edge of the patch apply with the grain going opposite from the back grain.

Monroe

The grain should run parallel to the top grain.

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Thanks for the info.

I remember reading somewhere that the patch should be off alignment (very little) with the top grain to get a strong reinforcement ??? I am confused. :)

How close does the crack need to be around sound post or bridge foot before considering a patch instead of reglue/cleats only?

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  • 6 months later...

Sorry to bring up this old thread. Most post cracks that I’ve seen mentioned here are found on violin tops (spruce), rather than backs. Unfortunately, I have a violin with a crack in the maple back only. Am I correct in assuming that gluing the crack and and cleating is not enough (even with the harder wood). I just want to be sure that I do this correctly, and that the procedure above is the good for the back repairs.

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Lots of people write stuff who don't know what they're doing.

Listen to Jeffrey and Jerry.

Yes, and lots of people do stuff who do not know what they are doing. Coincidentally, a friend just showed me a fiddle he recently acquired, that had an interesting repair. It is a very nice early American fiddle, with a rectangle soundpost crack patch, mortised out and patch set in, just like tile. The strange thing is, in and of itself, the work is actually very neat and professional (if only it was on something other than a violin), but of course it fails miserably from the structural side of things. How can someone do such clean work, and be so wrong about methods?

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iburkard,

I'm neither a repairer nor maker, but as a matter of what I've heard, I do not believe it's customary to fix a soundpost crack in a maple back with a patch, maple or otherwise, as one would the top; just glue and maybe cleats are used. If I've got the wrong idea, I'll be glad to hear corrections from the experienced restorers on the forum and discard this wrong idea.

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You will need to patch the back, just gluing and cleats will not work, they are on the wrong side of the pressure, the crack will just be pushed open by the soundpost pressure, the string pressure will hold the table cracks shut but the back has to have the pressure distributed away from the crack and the crack has to be bridged by the patch to hold it together, think of it as a hinge, cleating at the inside of the hinge will not stop it from opening

this is why the loss of value is greater from a soundpost crack in the back, they are more likely of future problems

use the same shape as it the phots above and make sure it covers the crack length, glue the crack shut with the back off the instrument and make sure the gap is closed, then cut the inside hollow, recheck the crack to see if the work on the hollow opened it, reglue and check hollow again, fit the plug and glue, Youmay have to make a plaster mold of the back or at least the c bout area inorder to get good glue pressure during clamping.

I just completed one, do you want some photos?

Reese

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use the same shape as it the phots above and make sure it covers the crack length, glue the crack shut with the back off the instrument and make sure the gap is closed, then cut the inside hollow, recheck the crack to see if the work on the hollow opened it, reglue and check hollow again, fit the plug and glue, Youmay have to make a plaster mold of the back or at least the c bout area inorder to get good glue pressure during clamping.

I don't believe the patch has to necessarily cover the full length of the crack. (What if the crack ran the entire length of the back?) But logically I think a back patch should be big bigger than a top patch simply because the back is thicker than the top in the sound post area. If the edges of the top and back patches slope at the same curvature then the patch will flare out wider and longer in the back than it will in the top, because the back is thicker.

I think some kind of mold is essential for fitting and gluing the patch.

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Brad,

good point about the length of the patch vs. long crack, I should have qualified my response to include a judgement of reason in that aspect, the eariler photos of the belly patch shows a nice combination of cleats and patch in that situtation.

Reese

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Ok, it looks like a patch for the back is an acceptable way (maybe the only way) to deal with a soundpost crack on the back.

What bothers me about using a patch for the back is that the last remnants of original, unbroken wood spanning the crack would need to be removed. At least on some back cracks, the crack is not visible on the inside, just the outside. So, however much unbroken wood remains on the inside spanning the crack, even if the "spanning wood" goes deep enough to be the majority of the back's thickness, it's got to go to put in a patch.

Is there any secure fix for the back crack that could leave at least some of the unbroken wood spanning the crack in place? Lifting an idea from the broken bow head thread, how about inserting 3 or 4 equally spaced splines, each maybe 2 mm wide, about 3 mm deep (the thickness of the back minus 0.5 mm), and about 30 mm long into the wood from the inside perpendicular across the crack. The splines would be inserted into exact fitting trenches perpendicular to the crack. In a back crack that's 40 mm long, 3 such splines, each 2 mm wide, would leave 34 mm of original, unbroken wood in place. Even if you had to double the number of splines to feel secure, you'd be leaving most of the original, unbroken wood in place.

I have no idea whether such of fix would hold, but at least such a fix would be more secure than just gluing the crack. The "spline" fix might also have the advantage of not needing a cast of the back.

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