Finally decided to abandon the old cup and cone bottom bracket and replace it with a sealed cartridge assembly and find myself needing some guidance. The old spindle/axle is 125mm long but I can only seem to find replacement cartridges that are longer or shorter by 2-3 mm. Should I err on the side of too long or too short? FYI, I did come across a few bottom brackets that measure 124-125 mm but the price, for me, was ridiculous at close to $200. I'm just a casual rider logging in around 200 miles per week and I'm trying to keep the price under $30.
Thanks in advance for any insight you can provide.

Depends on whether your current bottom bracket spindle is symmetric or not. You have to measure to see if the drive side is longer than the non drive side. If it is longer on the drive side, then a slightly longer cartridge will probably be needed

I would get a basic 122.5 one, (they basically give them away) and a few spacers, that will most likely work, and if not, you'll know what you need.

2-3mm is only like ⅛th of an inch. But you have to make sure you’ve got the clearance between the end of the crank, and the chainstay, and also that the crank will completely fit onto the BB spindle and not bind onto the BB shell of the frame. — Dan

I skipped cartridge type BB's all together. I just went from square tapered loose spindle with loose bearings and cups to a 2 piece crank and never looked back. I had a '78 Raleigh Competition GS with loose spindle BSA threaded BB shell and put a RaceFace Cadence crankset on it with their external bearing cups. The Shimano 2 piece cranksets are even easier to install on a BSA threaded shell.

You didn't say what type bike this is. Mountain bike, road bike or other. I don't think RaceFace still makes a road crankset.

Morning, and thank you for the quick response(s). Seems like this will be a work in progress once I get the new BB but I'll deal with any additional issues when the time comes. To start, I just want to get the most appropriate size and work from there - and the winner is...asymmetrical. The drive side is longer, so I should get the 127mm? Not to get too far ahead, but I'm assuming that the new BB will be symmetrical - so do I compensate by using a spacer under the drive side cup?

Just saw that last post by Iride01, and it's a MTB. I live in the Northeast and the winters make a mess of the roads - sand, gravel, branches, etc... The road crews usually don't finish clean up until late May so I use the MTB thru May and then break out the road bike.

Well, a number of cup/cone spindles were asymmetrical so spindle length is important, but knowing the DS length will give you the symmetrical cartridge replacement you need.

John

Just checked and the drive side is 3mm longer. So, I guess the 127mm BB with a 2mm or 3mm spacer on the drive side should do the trick, does that sound right?

I would purchase a Phil Wood BB. You can adjust the chain line even if they don't have the perfect length.

See equivalents

https://www.sheldonbrown.com/bbsize.html

...in your situation, I would go longer. A lot of this obsessing about an exact length for a replacement spindle is about maintaining a clearance for drive side crank arm, so there's no interference with the chain stay. But still keeping the spindle short enough so the non drive arm (and pedal) are close in for a tight "q" . In many cases, and for many users, how far out your non drive pedal ends up won't matter to you. And generally a tight "q" seems more to appeal to the road bike rider than mountain bikers. You might even discover you like the wider "q" better.

Even people who understand this topic pretty well sometimes need to go by trial and error fitting. That's cheaper and easier if you work in an environment that has some different used units to work with. Also, some spindle tapers will tend to push the crank arms out farther, depending on what your original spindle taper was. I think there's a section on Sheldon Brown that explains this.

The spacer thing works for adjustments for asymmetry of up to about 1-2 mm. But in my own experience, the less spacing you do with one of these, the better off you are. But I don't have some specific preference for a narrow "q".

Phil Wood? Too rich for my blood.
Hey dedhed, thanks for the link! Looks like 127 is the ticket.

​​​​​​​Phil Wood was the only one made that didn't break when I stood on the pedals years ago.

Bottom brackets and cranksets go together.

Cup & cone bottom brackets work just fine. Cartridge BBs, which I have used for decades, have no intrinsic advantages once the bicycle is assembled for the first time.

Bottom (bracket) line, there's no reason to replace a cup & cone BB with a cartridge one, unless you're also changing cranks, unless your existing BB is trashed beyond repair. Bottom brackets are like light switches... they're binary devices. They either work or they don't.

Overhaul your existing BB, and ride the bike for another 30 years. There's no improvements to be had, so save your money.

--Shannon

Edco Competition made a quality cartridge B.B. Often found on Ebay, link below is about 1/2 the price you got for Phil Wood but spindle is 121 mm here. But spindle can be moved laterally at least a few mm. So depending how much clearance you currently have between crank arms and chainstays, you might find 121 works. Otherwise longer spindles are probably available from time to time. And the price here is pretty good for N.O.S. compared to what's usually asked.

It is advertised for a triple set-up but I don't think that matters ( ? ) Also it's BSC threading if that's what you have. (Edit : Just read your post again - fails the sub $30 price limit).

NOS vintage Edco Competition bottom bracket adjustable 34.8 x 24 | eBay

OP is saying the budget is important. Phil Wood BB"s are not inexpensive

I am interested to know at what point a new BB is necessary. I have a rough cup and cone I am about to attempt to service. Does one replace when the races are so pitted that they are still rough after a clean, new bearings and grease? Do you replace when there is too much play from wear? Is it just up to the individual how rough is acceptable after a complete service?

Using standard machine repair practice ( e.g. auto transmissions ), if you had any significant pitting ( read visible to the naked eye ) in the bearing races, you would replace. Also, when a bearing is clean and lubricated, if the bearing/assembly is turned by hand, and any roughness is felt, or more than the slightest noise heard, you would replace. Bearings when new are supposed to feel silky smooth.

Moving to bicycle bottom brackets, I would think the requirements are far less stringent. The loading and speeds involved are really quite low. Some pitting could be tolerated as long as the bearing/assembly would turn easily and play could be adjusted to where it should be. I would think also some noise and even some roughness on turning could be tolerated. But if I could feel and hear this in the bottom bracket as I rode, that would irritate me, so I would replace in that case.

My Mexican and Chinese food delivery guys bring them in when the chain starts falling off. For the places that are good the repair is quoted in the number of menu items...