What do those ES numbers mean?

Removed the Octalink BB. I know the '68 X 113' and the '1.27 X 24' but what the !@#% does 'ES70' mean? And replacements are variously ES25, ES51, etc, and prices vary wildly. I have a vague feeling that the higher the ES number the better quality the steel??? Or is that EN I'm thinking of? Can I replace my ES70 BB with an otherwise-identical ES25?Help me Rhonda!
:?

Exactly, the higher the number, the better/lighter the bb. As long as you buy the right size it shouldn't matter too much. I think once you get higher in the range you get hollow axles and bolts.

Yep, the ES probably stands for something but actually means nothing, much like the UN in Shimano's square taper BBs.
The numbers do indicate the level of quality though, with higher numbers being the better BBs.
Octalink BBs and cranks were only really around for a few years before external type BB's became the new 'standard'.
I think Shimano still use the splined Octalink interface on lower end components though.

I'd go for the ES51 BB over the ES25 if you are looking for longevity.

Thanks for that. I'm guessing that ES25 is equivalent to (or became) Alivio, ES51 to LX and ES70 to XT ....roughly. Of course that doesn't explain ES30, ES71, etc nor does it tally with XTR being numbered BB-M95x!

I have figured out that 105 is BB5500 and Dura-Ace is BB7700 though but have also noticed that Octalink road BBs have a smaller spline length, so they're no good for my Preston.

Koobikes have ES25s on offer at the mo - if they last half as long they're still better value than a ES51!

M95x used the road spline length/spec of the ones you mention.
Octalink v2 didn't come out till quite a bit later.
ESxx just means Octalink Splined v2's
UNxx mean square tapers.
First number means quality level, second the 'era' or revision number.

Sort of, you know how these things actually end up being.

Back to ES (circa 2000)
70 is XT/LX
40 is Deore (and below if they used it way back when)
40 became 50 and eventually a 30 added, all deore level
71 was LX when XT became HTII
I think that was only for 1 year then LX became HTII and 30/51's continued in Deore....

You changed between 50 and 30 depending on the crank it is destined for, so probably a chain line thing.
Best to check the crank docs on shimano's tech site and see what it says it needs.