Terribly old fashioned I know but I'vr still got some in the cupboard. Am I right in thinking that the only difference between 68mm and 73mm square taper BBs is the none drive side collar/insert/doohickey thing? Got a 73 and a 68 and they look to my eye identical apart from the end stop on the NDS collar being 5mm different.
If thats right does anyone have and 73 NDS collars they don't need (or want to swap?) coz I've got a few 68mm BBs but only 73mm frames.
I'm not sure it's that simple. The shell length is printed on the body of these, so there must be more to it than the NDS cup:
I also seem to remember some BB's coming with a removable spacer on the driveside (or was that ISIS / Octalink?).
What 68mm's have you got that you no longer need?
although it's the non-drive side cup determining if the BB will fit into a 68 or 73 shell - the chainline must also be affected.
If you were to tighten the DS cup to the frame, then yes the DS taper would be 2.5mm too far to the NDS.
But if you put a spacer in youd have the right chainline, just that the NDS body would end 7.5mm inside the 68mm shell. If the NDS cup could be screwed in far enough to support the BB body, maybe it would work.
BUT UN54 BBS are so cheap, you might as well buy new.
Youll get a fair price for any 68mm UN72s you have to off load. Me first dibs! 
hmmm
ooh, I'll have one too, depending on the axle length.
Go on, you know you don't need them all.
Yes your right. The only differance is the cup. I've swapped cups before to change the BB length for a different frame and also bodged a 68mm into a 73mm with a spacer.
although it's the non-drive side cup determining if the BB will fit into a 68 or 73 shell - the chainline must also be affected.
That makes no sense to me. The cup is on the non driveside so how can there be any difference in chainline?
The SKF square tapers have a spacer so they can do 68mm and 73mm.
The relative position of the axle end and the DS flange must surely be different for any given axle length but different shell spec
I bought a Diamondback Apex Comp for Mrs BigJohn a few years ago and that had a 73mm BB in a 68mm shell. Not ideal because the l/h cramk was sticking out more than the r/h one by about oooh, 5mm.
But similarly, on my Soul I found that my r/h crank was too close to the chainstay with a 107 Sq Taper BB. Swapped it of a 110 and the r/h was still in the same position - the extra 3mm was all on the left. A 113mm was 3mm longer than the 100 on the right. Left stayed the same.
yup donk ur completely right,
its just the cup thats different,
go and ask ur lbs if they have anything knocking about
I think the chainline thing maybe a valid one now I've thought about it (it was making my head hurt so I went for a ride and thought it through)
IF the BBs are different then I presume they are something like this
- is taper
= is cartridge
! is BB shell
68mm
---!=====!---
73mm
--!=======!--
So if I fit a 68BB to a 73 bike it will look like
-!--=====!---
The BB would be off centre (but only by 2.5mm?), more on the DS, so the NDS collar will need to be flangless and screw in a bit further than it normally would.
Or as I first thought and others have concurred shimano could just be using the same BB with a different NDS collar.
Sorry if this is over complicated/unclear.
Firstly, are bottom bracket shells always sat central on the frame? Or are they ever offset? Does the drive side of the shell always sit in the same place? I'd say no, and that BB shells are always sat centrally, in which case the end on the BB shell for a 73mm BB will sit 2.5mm further out from the centre line than for a 68mm BB shell.
In that case, if you use a 68mm BB in a 73mm BB shall the chain line will be 2.5mm out, which you won't be able to correct with spacers.
Doing it the other way round may be possible using spacers, ie, using a 73mm BB in a 68mm BB shell.
Obviously, if my assumption that BB shells are always mounted centrally is incorrect, then this is wrong!
I've got two UN73s in a box in the garage. One is a 68/113 & the other 73/113. I've just put them side by side, ie lined up the DS flange and whilst the body is the same length, the axle lengths differ.
The 73 has a longer axle on the NDS and the 68 on the DS.
And yes the NDS cups are different, i.e the 68 is shorter.
I'd love a UN72/UN73 68-113 if anyone wants to get rid of one????/[selling?] Email in profile!
Thanks
No chance, 
, with 4 bikes with square taper chainsets I'm hanging onto them. Sorry.
I forgot to mention, this BB is for a singlespeed so should be able to get chain line sorted with the rear cog using spacers - thats assuming the cranks clear the chainstays. surely 2.5mm aint gonna be too bad.
BTW I tried a 68 with flangeless 68 NDS spacer, the spacer screwed a long way into the shell, so much so that the spline tool was close to bottoming out against the shell - that was hand tight, might not be enough space at the correct torque.
Whats the going rate for un72s BTW? don't make them any more do they?
Whats the going rate for un72s BTW
first-born child IIRC.
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