Electro-Harmonix - Sovtek Green Russian Big Muff π - knob

Sovtek Green Russian Big Muff π - knob
fit a 1/4" diameter, smooth, full-round shaft. If you are restoring an old pedal and the original pots are still good,
you can use the 1/8" adapter , to make the match.
Ream out the ends a little with a needle file and the adapter will slip right on