Safely making plastic knobs fit bigger pot shafts

Anyone here have any experience with SAFELY (meaning not to screw up vintage knobs that are rare) enlarging the hole in the knob to fit over a larger shaft?

The original plastic Fender F knobs are too small a hole to fit on the new electronics.
I want to retrofit them and use them, but sure don’t want to ruin them as they are super rare and hard to get.

I don’t have a drill press (maybe an excuse to buy one?).
I do have a dremel and a vice.
I often find working with the dremel in the vice and holding the part yields better results. I have a carving bit and two different size grinding/sharpening bits, both cylindrical that might do the trick if I go slow and steady, but wondering if anyone has better ideas?

I am ok with going a little too big, can always tape the pot shafts if needed for a friction fit.

I have some cheap ‘practice’ knobs to try on btw.


If they’re that rare are you sure you want to mod them? I think I’d be inclined to find pots with the narrower shaft rather than irreversibly mod a rare vintage part.

Plastic modellers use mini lathes to turn parts.
Perhaps a local model shop or modelling club could help.

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Three of the four are pots, one is the new buffer so no can do.

I got brave and did a practice run and answered my own question.

The bit on the right did the trick.
Dremel in a vice on lowest setting. Slight wiggle of the knob, check, wiggle, done.

Worked great, so I guess I answered my own question, but it works in case others want to know I guess.

Now to find a 4th knob to match the original 3. People on reverb want $100+ per knob. That ain’t happening.


For this I would use a modelling drill. Basically a screwdriver handle with a chuck for drill bits. Like a pin vise, but you can find them with larger handles for easier drilling. Here’s mine:

No way would I use any power tool.


That cutter wheel i used on the lowest setting was very very easy going.
Not enough power to do anything really bad. Very easy to control.

I would worry about the downward forces of the modeling drill vs. side widening.
The bit I used has no cutting blades on the top, so less risk of going through the top of the knob. (What I did learn later is the tops of these knobs actually pop off, which would have been smart to do before I started cutting too.

Clamping the Dremel and moving the part has worked out for me now really well in a few applications that could have gone horribly wrong fast if holding by hand.

Dremel does make a holder/press thingy to hold the tool and turn it into a drill press that would have probably been even safer, but I was too lazy to go get it.

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@John_E , I see you answered your own question. Definitely something like an end mill or a reamer (with a flat rather than conical or bull-nosed end) is preferable. Barring that, take a cheap drill bit of the appropriate diameter and grind off the tip, and then even up and sharpen the end edge of the flutes (DIY end mill for soft materials). I’ve done this for delrin and acrylic. It may work for wood and a soft metal as well (don’t know, this latter assumption is just conjecture).

Anyhoo, I wouldn’t recommend using the abrasive bits for plastic, because they tend to heat the plastic up, make it go gummy and stringy… in the worst case singe/burn.

Thanks! Hadn’t thought of hacking the end off a bit, great idea.
I like that little dremel wheel i used, perfect size and all.

I did quickly decide the grinding tips would only melt/cause a mess.