I got a cheap old bike I fixed up and I want to install a mid drive motor but the ones I found require a specific bottom bracket size and I don't know what size bottom bracket I have.

The wheels are 700 x 23

enter image description here enter image description here

  • Unrelated to your question - what is the big black rim on the front wheel - an electric light ? What is the "indicator stalk" thing in the frame ?
    – Criggie
    Commented May 7, 2016 at 22:33
  • On the wheel is Revolights. I got them for a good deal from a friend who runs a bike shop, they were a display model. The indicator stalk I'm not sure what you mean, if your referring to the clamp on the front, that's just a Cardellini clamp with a riser attached to it that I use for various purposes whenever I need to mount something to my bike which I do rather often. Commented May 12, 2016 at 6:43
  • 1
  • Thank you - those revolights look nice, but they wouldn't be legal here which explains why I've never seen them.
    – Criggie
    Commented May 12, 2016 at 10:16
  • Can you also provide a link to your planned BB motor? I'm curious.
    – Criggie
    Commented May 12, 2016 at 10:19

2 Answers 2


I found this diagram online which I hope will allow me to determine the size of the bottom bracket by measuring the width of the shell.

enter image description here

I found out that the bike is an old Centurion. I was not able to find any series number engraved on it anywhere so I haven't been able to determine the exact model or how old it is but just in case anyone wants to know; I concluded that the bottom bracket size is 125x 68 but that a 127 size also works. Thanks to the mechanics at IMartin in Los Angeles who took the whole crank assembly apart to measure the bracket when I asked them if they knew what size it was.


Based on that its probably a cup-and-cone bottom axle in English threadding 68mm. That's a very common size.

However the fact this lovely old road bike has a single-speed conversion and bullhorn hack/chopped handlebars, it could be a double or a triple bottom axle that has packers, or it could have been replaced with the correct length for the single-speed chainring.

I know its a conversion because I can see unused cable stops on the down-tube in your closeup picture.

EDIT: Next you need to get the pedal cranks off, and measure the distance between outside edge of the square taper shoulders. Probably 118 millimetres between, although 126 and 10_something and 97/98 are also possibilities. A common repair is to replace these cup and cones with a cartridge bottom bracket, fully watertight and good for 50,000 km of pedalling.

  • The bottom bracket is going to be standard. All I know is it has a square taper. I'm not even sure what brand the bike is, because it doesn't have any logo marks on it. I'm sure it probably did at some point but they're long gone. I've got a freewheel on one side and a fixed gear on the other side of the rear tire. I'm not sure what the bullhorn is. Commented May 12, 2016 at 6:46
  • @davidcondrey Bullhorn is the drop handlebars that have been sawn short. They're normally flipped over too to point upwards, somewhat like longhorn cattle.
    – Criggie
    Commented May 12, 2016 at 10:14

Not the answer you're looking for? Browse other questions tagged or ask your own question.