Bottom Bracket Sizing?

[pwpatton]

I feel like I should already know this but I don't...

So, how do I correctly size the bottom bracket for my bike?

I took the old one off and it doesn't list any sizing on it.

I measured the axle width and am pretty sure that measurement is 113mm. But I don't know how to take the 2nd measurement. I guess my 2 options are 68mm or 73mm... Help Please???

Thanks,
Phillip

[pwpatton]

I think I figured it out... I love the web. I'll explain what I learned in case others have the same question...

The 2 measurements are shell width and axle width.
The shell width is the width of basically what fits into the bike frame. It should be either 68 or 73mm.

I did find a number on the end of the shell that confirmed my measurement.... 68mm.

Here's a pretty cool length conversion utility that may help out.
<a href=http://www.boeckeler.com/metrology/length_equiv.html>http://www.boeckeler.com/metrology/length_equiv.html</a>

[egoo]

The best way is to measure the tube that the bb goes into. It will either be 68 or 73mm. Most newer mtbs have a 73. Get a shimano un72 if it's not splined, they last a long time, and they are lightweight. Should be about $40.