We were doing a volunteer bike repair clinic and came upon this bottom bracket that I've never seen before. It was on a trials bike and the crankset was destroyed but we couldn't find any way to service the bottom bracket and didn't have any matching cranks to replace it with. The bottom bracket was quite large and beefy and like like it would fit a one piece Ashtabula bottom bracket. Didn't end up fixing the cranks as we had no idea how to deal with this.

bottom bracket

  • 1
    The splining makes it look like a BMX 3 piece crank; if the size is ashtabula, is there an ashtabula to 3 piece adapter in place?
    – Batman
    Jun 2, 2018 at 21:24
  • @Batman It is some form of three piece crank. But this is as much as we could disassemble it. There wasn't anything we could find to remove after this point. Just a flat bearing cartridge and the spindle you see. Everything seemed oversized.
    – Kibbee
    Jun 2, 2018 at 21:55
  • It's possible that the bearings are compression fit. But I can barely make out notches on the outer ring about 1/3 of the way around. Did you try to unscrew the ring? Jun 2, 2018 at 21:59
  • Ran my finger around the outside and couldn't see anything to grip by. The notches you see might be damage. The spindle and crank seems to match the BMX three piece Batman linked to, bit the bearing appeared to be pressed in and non removable.
    – Kibbee
    Jun 2, 2018 at 22:02
  • @nathanknutson That answer looked decent - what was wrong with it ?
    – Criggie
    Jun 4, 2018 at 3:31

1 Answer 1


Looks like this is what's called a "mid bottom bracket". They seem to be common on BMX bikes. They use bearings that press into the frame. I found a video that shows how to remove and install a new one. It seems like the spindle should slide right out from the video but on the bike I was working on it didn't seem like that would have been the case, but I didn't really try to slide it out.


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