"How To" Cello Geared Peg Modification
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Violin Pegs / Cello Pegs have a tendency to slip, they are held in place via a method called friction locking, that is the wood to wood fit stops the peg from undoing and holds the string in tune. We use fine tuners attached to the tailpiece to tweak intonation.

A common modification is the fitting of geared tuners into the headstock, these come in many styles and varieties, the following tutorial is the fitment of a set of geared pegs advertised under the name of "Perfection Pegs"

Time to modify 1hr.
Remove strings and pegs.
On this specific type of peg, the manufacturer has locking threads, these are left handed and right handed, the purpose is once fitted it will resist string tension and undoing from its fitment.
We locate the relevant threads to the correct side and then mark each string position, this is important as each peg will be hand cut to its location.
Test fitting the peg, we can see the threaded locking section is a long way from engaging into the wood
Using a 2 degree tapered reamer we are going to open the peg box up.
Simply hold straight and turn a few rotations, it does not take much, better IMO to remove too little than too much.
Check the fit of the peg, almost there.
Another few turns.
Cello in for modification.
Original friction pegs and one missing
This is a geared peg, the internal shaft moves via gears, the outer section is locked into place
The correct fitment, will have just enough of a gap to allow the threads of the geared peg to cut a groove into the peg box.