What to look for in switching from square taper to octalink....?
 

Got a low end cross bike w/30/39/50 tooth crank and square taper BB. I want to switch to an Alivio mtn crank with octalink BB that I have lying around. Do I just make sure the width of the octalink matches the width of the original square taper BB?

Anything else I should be looking out for? thanks!

The bb spindle length is a factor of the crank, so as long as the bb and crank came off the same bike you should be fine. If you are not sure, Shimano's website should have the details for the required spindle length for those cranks. The only concern would be the bb shell width, the old and new should match, most likely 68mm. The spindle length will most likely be different between the two setups.

Two posts in nine months?

Dude, post more.

You'll see numbers like "68x113", where shell width is 68mm and the overall spindle length is 113mm.

These numbers are typically marked on the components. 68mm and 73mm are the most common widths.

Also, you'll need to make sure the bottom bracket threading is the same, which it probably is.

This guy knows way more than me, scroll down to "bottom bracket":http://www.sheldonbrown.com/gloss_bo-z.html

Also, remember to place grease or anti-seize on the bottom bracket to shell threads - this will make future disassemblies easier.

What are the tooth-counts of the Alivio chainrings? If it's a trekking crank with something like 48/38/28T, then it should work fine with the current front derailleur. If it's a MTB crank with something like 42/32/22T, then that's much smaller than the current FD was designed for likely leading to poor shifting.

Also, it's not a simple swap to go to a MTB FD because Shimano has different cable pull ratios between road and MTB front derailleurs and shifters; so if your road front shifter is indexed, it won't be compatible with a MTB front derailleur.

Square taper has a variety of lengths , Octalink has few,
but if you have an octalink crank it will be low profile, and determine the BB length.

another "standard" Shimano established, some others copied,
and then 'S' dropped it, and went on to something else.
and smaller companies were on the catch-up again.

In the interest of technical accuracy and minutiae, Shimano developed and patented the Octalink interface and then either refused to license it or made the licensing fee so high no one was interested, depending on which version you believe. The ISIS interface was designed specifically to get around Shimano's patent and was made public domain so any maker could use it.

Eventually Shimano did license a couple of other crank makers but by that time they were ready to introduce the external bearing Hollowtech II design and drop the Octalink so they had little to fear from the competition.