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Is there any difference between the main unit of 68 and 73mm bottom brackets?
Is it just the non-drive side section that is different?
yes, the NDS cups are the same, but the BB body itself is longer on the 73mm one.
You cant fudge it in a 68mm shell Im afraid.
If you've got the older UN71 type with removable cups the unit is the same you just need to 'transfer' the 68 cups for the 73's or vice versa.
With the newer style with integral d/s cups you could space a 73mm bb for use with 68 shell (2.5mm spacer, I'm good at maths) and hope that the non d/s seats adequately. It will sit inside the frame slightly. 68 bb to 73 shell won't go unless you machine the d/s face of your b/b shell....
I've got a 68 in a 73 frame, works, tho LH crank clearance is limited!
odd - I bought a 73mm bottom bracket, but the NDS simply wouldn't go in more than a few mill. The main unit seemed to foul on a lip of the NDS.
UN55 by the way.
I stuck an old random NDS adapter in and it fits fine.
Just checked (should have done this before I ordered), it should have been a 68mm. I must have got lucky with the old NDS fitting the new BB