replacement bottom bracket

insertnamehere said:

I already posted this in the cycling equipment section, but after having a better look around the forum, it seems like it might be better off in here.

I've had an Orange Evo 02 mountain bike for about three years now. The bike itself is at least six years old though. I used to take the bike to my local bikeshop for servicing and repairs when I was at university but now that I am working I decided to buy some tools and start servicing the bike myself with the hope of learling enough to eventually build my own bike.
Anyways, I have stripped the bike down and taken everything off except the headset and handlebars. The bottom bracket doesn't rotate particularly freely and is quite coroded on the outside. Part of the threads on the frame where the bottom bracket goes in are also coroded, but its only the last three threads and I don't think the bottom bracket even catches these threads.
So I want to replace the bottom bracket. The old one is a square tapered type.
Make: Shimano
Model: BB-UN40
Other numbers on the unit: 68 , BC 1.37 x 24

I would like to replace it with an Octalink bottom bracket but I think they are of larger diameter and won't fit into my frame. I have looked on the internet and a replacement for the old one only costs £12. I would like something a bit better than that. I will be upgrading to an XT drivetrain soon so an XT bottom bracket would be nice. What are my options here?
If you've read this far, thanks for your patience.

ok ahh the one you currently have the BB-UN40 part, UN is to do with the design of it, UN is most common, 40 is the quality and at a guess thats about deore level (cant remeber, shimano doesnt exactly make things easy with their codes) there should also be a code stamped on it like LL, a 2 letter code which is the length of the spindle an LL being 160mm. '68 , BC 1.37 x 24' - 68mm is the shell width (width of the part it goes into on your frame) the rest is the thread it has. so if you were replacing with another square tapered bb you would go get a BB - UN(number depending on quality you get) and with a spindle width of whatever the one you have now is, unless you want to change the chainline and a shell width of 68mm

going to a octalink BB shouldnt be any trouble, one WILL fit in your frame, just a matter of getting a 68mm shell and then getting chainline right

Thanx for the info. Well, after having a look around the net for a new chainset, I cant really find one with high enough gearing for me (I mostly use the bike for city riding and touring these days). So I've decided to just replace the bottom bracket and buy new chainrings for my cranks. I measured my old chainrings and found them to be 104mm and 64mm BCD. After scouring the net for parts once again I found that the outer chainrings seem to come in two different types.
Here are the two chainrings:
TA 104 4-arm outer 18mm 42 44 46 50
TA 104 4-arm outer 23mm 42 44 46 50
I know that TA is the manufacturer and the last 4 numbers are the number of teeth on the ring, but what is the difference between the 18mm and 23mm versions? The middle and inner chainrings don't seem to have any simalar sizing scheme.
Also is there a better model of my current square taper bottom bracket? I'm confused by the whole bottom bracket sizing and don't want to go wrong on this one.
Thanx for the help.