How do I find the right size of bottom bracket?

I have an old cup-and-cone system that I need to replace; for the sake of simplicity, I would like to replace it with a cartridge. Alas, I have no idea how to figure out what dimensions of the cartridge to buy; ubelievably, the exact dimensions of most parts don't even seem to be available at the point of sale, at least not online. This made me assume there were some sort of standardization to it, but if so I can't figure it out.

However, the last cartridge I bought (though the spindle length was within 2 mm) seemed a lot more wrong than 2 mm (I think the symmetry was wrong.) All I know is this:

SCM
3UB

is written on the spindle, which is about 124 mm long; and on the cup was written: 1.37x24T.

I have a triple chainring. Thanks.

• Your old spindle is likely asymmetrical. Measure the distance from the center of the bottom bracket/bottom bracket shell to the edge of the drive side spindle, double that number and you'll have your symmetrical cartridge BB spindle size. If you posted a photo of your bottom bracket and BB shell/bike we'd be a lot more able to guide you! Jan 13, 2016 at 8:55
• Yeah, with a triple chainring you almost certainly have an offset BB -- the right side shaft sticks out more than the left. Unfortunately, shaft length and offset is exceptionally hard to measure accurately. Jan 13, 2016 at 12:57

It would still be helpful if you posted a photo, but I think I can answer without it!

The first major dimension of a bottom bracket is its width. If you have a metric ruler or tape, simply place it underneath the bottom bracket shell. There aren't a bunch of standards super close together so you'll easily get this one right. 68mm is really common and would make your project very easy to complete, but it's ok if it's different.

Like @Criggie said, the 1.37x24T measures the threading. Threads are the next most important dimension, and yours is standard English which is very easy to replace.

Like I said in a comment, your old spindle was probably asymmetrical, but new ones are typically symmetrical. To get your correct spindle length, you need to measure from the center of your spindle to the outer edge of the drive (or right) side. Double that number and you'll have the correct new spindle size. You'll probably want a pair of calipers for this, but the \$5 ones at the hardware store are more than sufficient.

I did a quick google and your bottom bracket is probably a Sugino. All I did was type in "SCM 3UB bottom bracket" to get there. This is a Japanese part that is likely NJS spec'd (criteria for use in Japanese track racing) so it very well could have an ISO taper and not a JIS. See here from Sheldon to understand these terms. If your measurements are perfect and your crank won't slide on to the right place, this is the next culprit. If this BB is ISO, and your replacement BB was NJS, then your crank arm was probably about 4mm off from where it needed to be.

Did you already get the fixed cup out of the bottom bracket shell? If you didn't, I'd just take it to the LBS for help. They can do all of these measurements (including checking to see if it's NJS or ISO) for you, and that way you only buy one bottom bracket- the one that fits.

Good luck!

• If the new set is symmetrical, how will it replace an asymmetrical one? Jan 17, 2016 at 21:01
• It would be a couple mm off on the left side potentially. Phil Woods makes a pricey adjustable BB, but we haven't solved the taper mystery for their cranks so recommending anything specific is difficult. Jan 17, 2016 at 21:11
• When I last bought a cart, Phil Woods made several offsets for my size (plus the cups were adjustable within about 2mm). Jan 17, 2016 at 21:13
• I can revise the answer a bit. Thanks for the heads up. Jan 17, 2016 at 21:14

1.37 x 24T is the big threadings, its 1.37 inches across and 24 threads per inch (so you need to turn it 24 times to advance it 1 inch)

Also check Saint Sheldon's info at http://www.sheldonbrown.com/bbsize.html
Based on that I guess you want a 3NN bottom bracket cartridge, but do use a ruler/calipers to confirm all the dimensions.

Your other option is to clean and take the old spindle/axle into a nice LBS and ask for assistance. They're generally good at that kind of thing.