Norco 250 bottom bracket sizing

[Fat Whacker]

I am currently building up a 2002 Norco 250. Most of the parts are coming from previous bikes that have been upgraded at one time. All the parts are still good, but have just been sitting in a box.
I was originally going to pick up a nice 3 piece BMX crankset for it as it had a bmx sized bottom bracket. But after some thought, I have a pair of Shimano cranks that I stripped down to just a 42 tooth ring. Basically the same thing as a BMX crankset. So I am just going to use the bb reducers and a mountain bike bb.

Now my question is this. When I am looking at bb sizes, what is the size I am looking for? For example, what is the difference between a 110mm and a 120mm? Is it the width? The circumference?

Sorry if it seems like a noob question, but when it comes to bb's I am not to knowladgeable.

[switch]

I don't know much about BMX type BBs, if that is what the 250 has.

For MTB BBs, you have the width of the bottom bracket shell (where the bottom bracket goes into), and the width of the spindle. Typically, MTB bikes have either a 68mm or 73mm BB shell width. Spindle width is typically 113mm or 118mm, but ranges from 108mm up to something like 140mm.

It's sounds like you're talking about spindle width.

Your Shimano BB is an MTB BB, so you need to make sure the 250 frame takes an MTB BB (or you might be able to use a reducer?). Then you need the correct BB shell and spindle width. Most MTB BBs are built to work in a 68mm or 73mm BB shell (a spacer is provided). Spindle width is used (typically) for chainline.

[Fat Whacker]

I am thinking that I should probably use a bb with a narrower width because of the Shimano cranks I am using. They are just a regular pair of cranks that used to have 3 rings and is now down to just the largest ring. So the chain will be out a bit than if it were a regular bmx crank. So to compensate, I am thinking a narrower bb would help.

Decisions. Decisions.

[switch]

I'm surprised no one else answered your question. The 250 is a pretty popular bike.

What you want is the best, or straightest, chainline you can get.

Are you using a rear cassette and derailleur, or is the bike single speed?

[Fat Whacker]

I was thinking about going single speed, but for now I have a 9sp cassette and derailleur. I will probably go ss in the future when more $$$ come available. I don't want to spend all that much $$$ getting it set up because I have 90% of the parts for it. The only things I need are the headset and bottom bracket.

[switch]

The chainstays aren't really long, so using the largest cogs will create a skewed chainline, but it probably won't be a problem.

If the frame takes an MTB BB, then your Shimano BB should be OK.