Thompson Belly Reducer

  • 11 Reviews
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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

Item # 1380
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Quantity: 20


T.J. Thompson explains the inspiration behind the Thompson Belly Reducer

Save the bridge plate and save the value of your vintage guitar! T.J. explains the inspiration behind the Thompson Belly Reducer and more in his interview with Dan Erlewine at the 2010 NAMM Show.

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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.

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.

Trade Secrets! Newsletter

Fixing a potbelly with the Belly Reducer

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...

Product Instructions

Thompson Belly Reducer Instructions

The Thompson Belly Reducer is a simple tool designed to get rid of the bellying without making permanent alterations to the original guitar top.

Ratings & Reviews

Thompson Belly Reducer

  • Based on 11 Reviews
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Thompson Belly Reducer

By Steve Long (UK) from Flitwick, Bedfordshire, UK
(Customer's Reviews) Wednesday, March 26, 2014

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.

Skeptical at first, but..

By Tim Gospodinov from NYC
(Customer's Reviews) Tuesday, February 04, 2014

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 :((((

Thompson Belly Reducer

By MED from Delray, FL
(Customer's Reviews) Monday, August 05, 2013

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.

Too soon to tell.

By BKJ from Va
(Customer's Reviews) Sunday, May 27, 2012

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.

less regrets (can I get a time machine?)

By Pat DiBurro from Exeter, NH
(Customer's Reviews) Tuesday, November 08, 2011

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.

Fantastic Tool

By Joe Konkoly from Lansing, MI
(Customer's Reviews) Monday, September 26, 2011

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.

Extremely effective, this tool kicks belly!

By vle from USA
(Customer's Reviews) Monday, September 19, 2011

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...

worked perfectly

By Steve Kovacik from Scotia, NY
(Customer's Reviews) Monday, September 19, 2011

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.

Belly Hard

By MED from Delray, FL
(Customer's Reviews) Thursday, April 28, 2011

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.

Effective but awkward to use

By Martin Pollard from Sussex UK
(Customer's Reviews) Saturday, February 19, 2011

Apart from the construction of acoustic steel guitars I now and again get the opportunity to repair vintage ones. Most require neck reset and bridge problems resolved. I've used the TBR on a 1931 Gibson LO and various Martins including circa 1890 2 1/2- 18, 0-28 and a 1926 0-18.

The difficulty I found is getting the inner plate in position while hot and keeping it there until the top plate is firmly clamped. Thin gloves help, the use of thick gloves is just prohibitive. Where bellying is behind bridge area on straight bridges, usual, the use of the belly bridge clamp has proved effective. Inner clamps would benefit from either a removal handle or built in positioning clamp.

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