Bicycle Mechanics - How to convert square to octalink or Issis

I have 2 bikes with standard square 68x113 and 68X122.5 BBs both with Shimano 105 square cranks. How do I select the correct Isis or Octalink BB and crankset? Is there a web site with a chart?

http://sheldonbrown.com/bbsize.html

I have 2 bikes with standard square 68x113 and 68X122.5 BBs both with Shimano 105 square cranks. How do I select the correct Isis or Octalink BB and crankset? Is there a web site with a chart?

Why change, been using the square for ages, cannot see the benefit in changing

I have 2 bikes with standard square 68x113 and 68X122.5 BBs both with Shimano 105 square cranks. How do I select the correct Isis or Octalink BB and crankset? Is there a web site with a chart?

Depends.

The BB spindle length is determined by what crank you are using. The cranks that you have now, with the square hole, obviously won't work with the star shaped ISIS spindle. The 68 part refers to the width of your bike frame's bottom bracket shell and equally obviously won't change.

Yup. As R.G. says, you'll need a 68mm (English-threaded) BB shell, but other than that you need whatever spindle lengths your new cranks require. You've noticed that different versions of Shimano 105 square-taper cranks (I assume both are double cranksets - it would have helped if you'd specified in your post) require different BB spindle lengths for correct chainline.

ISIS is more straightforward - nearly all ISIS cranks are built to the same standard, so road doubles ideally go with 108mm, road triples with 113, and off-road triples with 118mm (b/c off-road rear hubs are 5mm wider). I'm not sure about Octalink.
But jur pointed you toward Sheldon's BB-length database, which should take care of you here.

I don't actually think that it's worth your changing to an ISIS crank, though, unless you've got a good deal on one and one of your current cranks needs replacing. Square-taper spindles are slightly less stiff, in a way that's basically unnoticeable by the vast majority of riders. Square-taper sealed-cartridge BB's also have a much better record for durability than splined BB's, because the larger BB spindle in the latter leaves less room for bearings, meaning the bearings are smaller and subsequently less durable.

If you want more of an answer, say more about your situation.

Square-taper spindles are slightly less stiff, in a way that's basically unnoticeable by the vast majority of riders.

+1 All the hype on ISIS, Octalink and one piece cranks are all a bunch of hype. If Sydney was here, he would tell it like it is.


Tim

thanks. The BB of my road bike is the old style where you replace the ball bearings. I need to service it and was thinking of going with a sealed BB. If I am going to go with a sealed,maybe I should upgrade, I have seen cranks + BB for $99 on sheldon brown.

The other bike is a 122mm and I guess I will have to stick with square one way or the other

You can get a square taper cartridge BB for 20 bucks. If the cranks aren't broke, don't bother changing them too.

You can get a square taper cartridge BB for 20 bucks. If the cranks aren't broke, don't bother changing them too.
Yup. You can get a Shimano UN-5X or UN-7X in 122.5mm. Other makers (Phil, FSA, Sugino, etc.) as well. Though your best bet here is probably Shimano. No reaspon for you to get a splined BB.