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1920s Gibson F-4 Mandolin
Tuning Gear Repair
Or, "Why repair guys save every little part."
© Frank Ford, 6/29/02; Photos by FF

Some time ago the cog was lost from one of the "D" string tuners and was replaced by one that looked like a pretty good fit.
The only real problem is that, while the gear actually worked for a while, the teeth didn't actually mesh correctly.
Because the gear was a bit harder than the brass worm, it was the worm that was destroyed after a few years. You can see that the teeth were terribly deformed, so that no gear replacement would be possible.
I took my flush cutting pliers and coaxed the peened-over rivets out of their holes.
They're not actually rivets, but the machined ends of the little bearings that hold the shaft in place.
A bit of heat allowed me to slip the button off the shaft.
The button survived the release (most of them do) and was ready to press onto the new shaft.
Now, here's the painful part. It took about two hours to come up with the ideal donor. Even though my salvage parts are well arranged and sorted, it always seems to take longer to find one that will actually fit. In those days, the machining just wasn't up to today's CNC standards, and manufacturers seemed to do very small batches before changing tiny specifications, or were downright sloppy in their machining and assembly,.
In order to make sure I had enough "rivet" material, I sacrificed the plate of this old set to get the worm off safely. I felt it was important to transplant all the parts - worm, cog, bearings, and screw - as a unit so they would have the best chance of working well.
Looks like there's just enough.
For a tiny bit of extra security, I chamfered the back side of the holes in case the rivet section proved a bit short.
I rummaged around and found a drill bit whose shank was a perfect fit in the hole in the bearing that was loose from the shaft. When I peened over the rivets I didn't want to distort the bearings, so I counted on the drill bit for the first bearing and the worm shaft for the other.
My new rivets aren't quite as neat looking as the original, but they seemed quite solid.
The set looked good after I pressed the button on the shaft.
Some of that famous elixir, OIL.
And, I'm pleased to say, the "new" tuner works as well as the originals.

The bad news: because repair charges have to be based on time, this can be an unpredictable (and expensive) job, what with the difficulty in finding just the right transplant parts.

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