I'm going to swap out some old cottered cranks for a new set of cranks.

So, I have to remove the cotter cranks, and the bottom bracket.
Then I'll need a new bottom bracket with square tapers and a new crankset with square tapers, and thats it, right?

For the bottom bracket, do I need to measure the width of my frame's bottom bracket shell, or are they more or less universal and thats what the adjuster cups are for?

Thanks very much!

Most important is to take note of which direction you have to turn things when removing the existing BB, in order to determine if Italian or English thread (hopefully not French or other standard). You can measure if you want, I suppose. 68mm= english, 70=italian

Check with new crank manufacturer to determine appropriate BB spindle length, however. I have some Campagnolo doubles that require 102mm and some Shimano triples that like 113, for instance. Wrong size may throw off your chain line.

It best to know the whether the BB is Italian before you go trying to remove it. The difference is with standard ISO or English BB, the drive side cup is reverse thread but an Italian is not (the non-drive side of both are right hand threaded). I'm guessing your BB has been on the bike for a while so you may not be able to tell right away which direction loosens the drive side BB. If you can measure the BB shell and feel comfortable making a 2mm call go with it. If not, what is the make and model of your bike if you know it and what make of components do you have?

So, if I buy a new bottom bracket, will I not be able to use the existing cups that are already there?

I thought I could just take the cups off, insert a new (square taper) spindle piece, then put the bearings/cups back on.
Essentially just turning the ends of the bottom bracket from cottered to square.


As far as the make/model, its a weird old little folding bike thats been repainted (not by me) and I really don't know much about its origins.
Just that its a single speed w/ a coaster brake.

Also, would it be possible to just take a used bottom bracket/crankset off an old 10 speed I've got lying around and just swap that out?

Thanks again for all the info!

Yeah you can just ditch the cottered spindle and replace it with an appropriate square taper spindle and crankset (using the original bearing cups.) If you do this, do not bother removing the (driveside) fixed bottom bracket cup. Especially since you don't know which way to turn it.

If you already have a crank and BB to use, try it and see if it fits with the cups you already have in the bike. Otherwise, you will need a new square taper spindle and crankset.

+1 Measuring the BB shell will help. 68mm, 70mm, or 73mm?

Unless you have a really good light, it may a wise idea to go ahead and remove the fixed cup for inspection if the "old" bike hasn't been serviced in a while...

I just did this - replaced the cottered spindle with a square taper spindle. Took the cups, bearings, and old spindle to the bike coop and looked through their box of old spindles. Cost $8.

Now, I did that because the bike had a French thread bottom bracket and a cartridge BB for that thread is $50 + shipping. If I had standard English threads and thus the option of a cheap standard cartridge BB, I'd do that instead.

Just checking, it's not a Raleigh is it? If it is, there's a chance the BB thread will be Raleigh standard rather than english standard (26 t.p.i. rather than 24)

If this helps when changing out cottered old crank spindel for a newer square type it seems better to use new loose ball bearings rather than raced while a little more tedious it just seems to work better.

Since no one has asked so far, I will. What's the make, model and approximate age of the bike? Is there anything engraved on the current bottom bracket cups that might tell us what it is?