I am fixing up an old MTB BSO (MTBSO? Or even MBSO?). I reckon it's from the 1990s or early noughties. My ambition is to do it myself and as cheaply as possible. The fault is that the axle snapped and some of the loose ball bearings were lost.

I've had a read of this: https://bicycles.stackexchange.com/a/43295/33048, in which the need for an educated guess is identified. I am hoping someone can help me make that guess.

The problem

The parts are not what I was expecting:

  • the nuts are odd – I suspect it’s just terrible quality, but maybe it’s something else:
    • the edges seem sharper, or more pronounced, than I am used to
    • the nut sizes are funny: I can barely get a size 15 spanner on, then size 16 has lots of play
  • the bearing balls are 3/16” and Sheldon Brown says 1/4" is normally used: https://www.sheldonbrown.com/harris/bearings.html

Rough, tentative plan

  1. For the axle, my plan is to replace it like for like, I have no appetite for splashing out on a quick release.
  2. For the bearings, my plan is to replace the existing loose ball bearings with cage bearings. I know loose ball bearings are better but here are my reasons:
    1. the bike is only for light, occasional use;
    2. I've once replaced a caged bearing with loose balls and found it very frustrating.
    3. I am struggling for time.


  1. I can’t tell what size caged bearing I should buy to get it right first time and only buy what I need.
  2. Shops seem to market their caged bearings as one of the following:
    1. “3/16" Front Hub Caged Bearings” – I assume my axle (the existing and the replacement) is 3/4" = M10, so the concern is I might struggle to get the axle through the bearing, let alone the cones.
    2. “1/4" Rear Hub Caged Bearings” – my concern is that the cup in the hub will be too small to hold this, and I don’t know how to measure to be sure.


It seems the wheel meets one of the following descriptions:

  1. The factory used the wrong size balls, but otherwise everything is normal: the axle is 3/4" = M10, and the cups in the hub will fit one of those “1/4" Rear Hub Caged Bearings”
  2. It is a rare-ish but reasonably standardized (not unheard of) system where I can use an M10 axle and a “3/16" Front Hub Caged Bearing”
  3. It’s some weird, non-standard thing and it’s going to be pot luck, hit and miss

Main question requiring an answer

Which description from the “Speculation” section above is my wheel likely to meet, if any?

Supplementary questions - opinions would be appreciated but are not essential

I know I probably could find the answers to some of these if I put more hours in, but hopefully I can ask them here.

  1. Is my general plan reasonable? Or am I overcomplicating it?
  2. Are there any measurements I can take on the hub’s races to narrow down the search?
  3. Will the cones fit? Can I buy a new axle complete with bog standard cones, and will they fit in the caged bearing, whether I go for 3/16” or 1/4"?
  4. Will I even be able to get a caged bearing into place through the freewheel?

Some photos

Please excuse imperfect focus. existing cone 1 existing cone 2 view from drive side view from non-drive side 1 view from non-drive side 2

  • You will have to remove the freewheel to install the bearings and axle easily. Stick loose bearings in place with thick grease -- it's an easy job and they won't fall out while you are messing around. Car/trailer wheel bearing grease works well, very thick.
    – Noise
    Commented Apr 12, 2022 at 8:30
  • Try to get replacement axles in bulk... they break absurdly easily on those bikes. Commented Apr 12, 2022 at 13:06
  • @leftaroundabout Thank you but I don't think I understand. Is there something about BSOs that makes axles break more easily? I can think of several factors affecting the life of an axle. But it didn't think it mattered what the quality of the remainder of the bike was.
    – pateksan
    Commented Apr 15, 2022 at 20:03
  • @pateksan yes. The problem is that these bikes have a freewheel that requires the axle to poke out completely unsupported for quite a long distance. The resulting lever makes it so even moderate impacts on the rear wheel bend the axle slightly. Now, that's not too bad in a road bike which isn't really supposed to take any hits ever, but in a mountainbike... come on, you know you're going to do the occasional hop or smash into some rocks, and after a couple of times the axle inevitably just snaps then. Commented Apr 15, 2022 at 20:49
  • @leftaroundabout Fair enough. It crossed my mind you might be thinking about any bike with a freewheel. But it didn't seem right as there are a lot of bikes with freewheels which I wouldn't call BSOs. But I admit I stopped keeping up with bike technology some 15 years ago, and I guess I need to accept that things are changing, which I guess is good in the grand scheme of things.
    – pateksan
    Commented Apr 15, 2022 at 22:56

1 Answer 1


Unless you are lucky, the time taken to install loose bearings will be less than the time needed to track down a caged bearing. Cost wise a 'bucket' of loose bearings is the cheapest approach. As such, I would replace like for like and save a whole raft of potential issues, i.e. I suggest loose 3/16" bearings.

Installing loose bearings is not hard, but it can be frustrating the first few times, I don't think is a good reason to avoid them on this job.

Those cones don't look in great shape, if the cups are good, I would look to installing new cones. Getting ones that fit could become and exercise in trial and error (with bearing size coming into it), so may be a reason to stick with the cones you have. This kind of job - occasional, light use and lowest cost requires being pragmatic, when I have done it for myself, I accept possible rework down the line if I cheap-out too much. If doing for someone else, it get more professional and put in the new parts.

  • 1
    Thanks. I have to admit I'm really put off loose bearings. I think I will wait a bit longer to see if I can get a replacement wheel somewhere, I'm not in a hurry with this bike. Sorry for any disappointment.
    – pateksan
    Commented Apr 15, 2022 at 20:07

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