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Home : Tools : Clamps, support tools : Thompson Belly Reducer

Thompson Belly Reducer

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A lifesaving tool set for older acoustic guitars! Helps correct soundboard bulge caused by string tension, without replacing the bridge plate.

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Thompson Belly Reducer

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1380 Thompson Belly Reducer Yes

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Excessive soundboard "bellying," caused by string tension, can make an acoustic guitar difficult to play. It can even damage the top, bridge plate and bridge. Replacing the original bridge plate to correct the problem is a time–consuming job that can unfortunately reduce the value of a vintage instrument.

Respected luthier and restoration expert T.J. Thompson developed a simple tool set, the Thompson Belly Reducer, that can fix soundboard bulging without permanent alterations to the top or bridge plate.

The Thompson Belly Reducer is a set of five precision-machined aluminum tools: convex flatteners for vintage–style belly and rectangular bridges, and concave flatteners for the three most common bridge plate shapes.

They're designed for use with the bridge removed from the instrument, and are effective for bridge plates that have been installed with hide glue, aliphatic (yellow) glue, or PVA (white) glue. Simply match the appropriate flatteners to your guitar, heat them to 150 degrees F., dampen the wood on the bridge area and bridge plate, and clamp the flatteners into place for a day or so. Instructions are included (view instructions online).

T.J. Thompson recommends using three Ibex bridge clamps.

Fixing a potbelly with the Belly Reducer
Fixing a potbelly with the Belly Reducer
Fixing a potbelly with the Belly Reducer Read more

In this issue:
Dan Erlewine’s got a Martin with a potbelly: string tension has pulled the top into a hump on this 1928 0-18. To make things tougher, the guitar top’s encased in a thick layer of epoxy finish! It’s an interesting puzzle...

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 Ratings and reviews
Thompson Belly Reducer
Thompson Belly Reducer
4 out of 5 stars
Based on 13 ratings Write a Review
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4 3-26-2014
Thompson Belly Reducer

Bit of a pig to use if you have big hands (like me) and can't get both your arm and the first clamp into the soundhole at the same time but if you can get it in place whilst it is still hot and the wood wet, it works a dream! Worth persevering with, it really does work. All else fails, try getting someone to hold the guitar soundhole down to get the hot inside caul in place.

Steve Long (UK)  Flitwick, Bedfordshire, UK
5 2-04-2014
Skeptical at first, but..

It actually works quite well. It took me some time to make some cauls for the Ibex Bridge Clamp to fit over braces, but other than that pretty simple and straight forward. I have to ask though -- WHY did you switch to black finish? I really liked the polished aluminum. With these every little bang against other tools or surfaces leaves a white mark :((((

Tim Gospodinov  NYC Read my reviews (8)
4 8-05-2013
Thompson Belly Reducer

This tool is a great concept; however, using it is a puzzle. The video interview does not explain use and nowhere are there any instruction on how to apply this. It would be great if Dan or Erick would make a brief video on how to properly use this tool.

MED  Delray, FL Read my reviews (37)
3 5-27-2012
Too soon to tell.

Ordered this product to repair a severely warped and bellied vintage Gibson J-35. It didn't work very well. Perhaps the guitar is too far gone. Maybe it works fine on normal bellied problems. I'll see soon, I hope. The directions (to me) are very basic, and do not give enough information. Good directions (to me) consists of possible problems and possible cures. I've not given up on it yet however.

BKJ  Va Read my reviews (3)
5 11-24-2011
Over the hump...

The TBR really got me "over the hump" on a couple of Martin 0-17's I made improvements to. Wear thin gloves, do a dry run, follow instructions, and prop the clamps as Dan suggested in a recent "Trade Secrets" article. That article really shows the proper use as well as tangible results. You'll notice the improvement too. I envision other uses for the TBR as well. Thanks, TJ!!

John W. Keane  Prior Lake, MN
5 11-08-2011
less regrets (can I get a time machine?)

As the pricing of vintage instruments increases, the more we as craftspeople touch an instrument the more our work can negatively affect the value. Buyers are becoming increasing aware of bridge plates and often devalue a instrument based on its replacement. A bridge plate removal also offers many opportunities for unwanted outcomes even for advanced repairers. I've been using the Thompson designed Belly Reducer for about a year and can report back that it has spared many instruments from the slice of my plate pulling tools.

A couple tips that may already have been brought up in the instructions or previous reviews. Perform a cold dry run first to be sure you have the necessary clamps and general feel of how you will set the cauls in place. It's a lot like working with Hide Glue, time is critical. I marked my cauls with a sharpie to denote treble side and also up and down positions to save a few seconds after they came out of the hot pot. This tool is most functional on bridge plates that closely match the footprint of the cauls so don't expect miracles with an oversized plate. Now if TJ could design a time machine so I can back and heat straighten all those old bridge plates I yanked.

Pat DiBurro  Exeter, NH
5 9-26-2011
Fantastic Tool

I have using the Thompson Belly Reducer on numerous guitars for the past year. Results have been excellent on hide glued instruments. The precision machining of the pieces matches the Martin vintage stuff really well. Good results with PVA glued plates and helpful with various other applications too.

Joe Konkoly  Lansing, MI
5 9-19-2011
Extremely effective, this tool kicks belly!

Seriously, I have used this tool on two vintage guitars which would barely have been salvageable without it. Excellent results, and the repair increased the value of the guitars, which wouldn't have happened if I removed the bridge plates. If you carefully follow the directions it's as easy to use as hot hide glue. Very glad I have this tool! Now if only stewmac would sell one that would flatten mine...

vle  USA
5 9-19-2011
worked perfectly

I have used the Thompson Belly Reducer on several vintage Martins with excellent results on all of them. In each case I followed the instructions as written and left the piece clamped for one week. The tool left each top very flat. I had little trouble with placement or with the plates cooling too fast during placement. I used leather gloves to keep from burning my hands and had no problems.

Steve Kovacik  Scotia, NY
3 4-28-2011
Belly Hard

I like this tool's concept, but the application is an issue as can be read in other reviews here. I'm hoping StewMac will develop a process to apply the inside plate.

MED  Delray, FL Read my reviews (37)
Displaying 1 – 10 1  2 > /\ Top of Reviews

Shown on page 28 of our latest catalog.
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