Chuck keys - how are they sized?

[Chris Green]

I need a chuck key for a little pillar drill's chuck, it's a 6mm capacity chuck. I'm not sure if I never had the chuck key or if I lost it but I need a new one anyway.

So, how are chuck keys sized? Looking on eBay they *seem* to be for

1/4", 3/8" and so on chucks. Thus I would appear to need a chuck key for a 1/4" chuck but I'd like to be certain.

[John Rumm]

They normally specify the diameter of the pilot (typically 3.2, 5, 6 or

8mm) and the pinion gear size - both diameter and number of teeth.

If unsure, you could by a combination key - they normally have 4 ends in different sizes.

[Chris Green]

Thanks John, my little chuck has 6mm holes for the pilot. I guess I can estimate the size of pinion and then at least I have an approximation to what I want.

Yes, I saw them when I was searching initially, they get rather mixed reviews.