Thompson Belly Reducer

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Thompson Belly Reducer

Item # 1380
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Thompson Belly Reducer

About This Item

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 TJ 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.

Aligning the cauls is now easier—magnets are embedded right into the tools. Magnetic cauls stay in place (hands free!) while installing and tightening the clamps.

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 included.

TJ Thompson recommends using three Ibex bridge clamps.


Trade Secrets!

Trade Secrets!

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.

Customer Reviews

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A Most Excellent Tool

(Customer's Reviews)

I've been using these tools for quite some time now on different model guitars with bellying problems and i'm very happy with them. i have the early models without magnets. To position the inside one i insert it cold and mark the desired spot with a permanent marker through two bridgepin holes. I then prepare a wooden post with a string attached to it that will temporarily hold the inside caul once it's heated up. The string hangs outside the sound hole. i then place the guitar face down with a moistened sponge on the bridgeplate for at least an hour, while i heat up the cauls. Once they're hot enough and the bridge plate glue is malleable i insert the inside caul with the wooden post which gives me time and space to place the top caul and insert the clamp. pull the string to remove the tempoarary post before tightening the clamp!


Thompson Belly Reducer

(Customer's Reviews)

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.


Thompson Belly Reducer

(Customer's Reviews)

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.

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StewMac PDF Catalog, page 30 See Thompson Belly Reducer
on page 30 of our StewMac Catalog PDF

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