I was going to purchase bearings soon and i was wondering about some different sizes.

I have one of those park tool rulers with the ball bearing size on it and so far i've determined:

- 1/4 = rear hub ball bearings for shimano and other brands or bottom bracket race bearings

- 3/16 = Front hub bearing for shimano and some other brands

the next few i haven't figured out what they're used for, hopefully you guys can fill in the blanks

1) 7/32 ?
2) 5/32 ? (headset bearing?)
3) 1/8 ?

7/32 for Campy front hubs (many)

5/32" most pedals
1/8" most of the hubs I have used

5/32 for headsets.

1/8" for freewheels, and freehub bodies that don't use a cartridge bearing.

Vintage Campagnolo "Record" front hubs, Zeus 2000 titanium bottom brackets.

Many pedals, some headsets (e.g. Campagnolo Record track and Gran Sport road, many Japanese and French headsets), Helicomatic rear hubs.

Freewheels. Some pedals.

3/16: many front hubs, including Campagnolo Tipo, Triomphe, Victory. Vintage Campagnolo Super Record titanium bottom brackets and C-Record bottom brackets. Also some obscure 1960s Campagnolo Record bottom brackets marked "CON SFERE DA 3/16" Some high-end headsets like Campagnolo Record and Super Record, Shimano Dura-Ace and 600, etc.