This is a reduction dial for those small plastic variable capacitors. Vernier dials are hard or expensive to come by, but they're so simple. So I made this.

It uses an 8mm bolt, and an o-ring. The o-ring I used works but could be thinner. You can use an o-ring, a rubber band might work, maybe some string if you wrap it around the bolt shaft a few times.

The knob was designed to fit the head of the 8mm SHCS that I had. So it may not fit yours. It has a little 6mm hexagon inside it to lock it into the head of the bolt.

I made a bottom mount, and a panel mount version. Both have 4mm bolt holes. My variable capacitors use 2.5mm bolts, yours may be different.

I will likely make a version for air variable capacitors soon, as I have some in the mail.