Bicycle Mechanics - Will a 68mm square taper BB fit into a mtb frame that is 73mm english threaded?

Will a 68mm square taper cartridge BB fit into a mtb frame that is 73mm english threaded? Both the bb and the frame are english threaded, so would it be a big problem?

...the reason why I want to do this is purely to figure out some chainline measurements with an old crank that I have. I have a few old 68mm cartridge BBs sitting around, and just wondering.

if the bb cups are flangeless, yes

http://www.philwood.com/BB-Too7.jpg

You say you have a frame with 73MM and you also say you have 73MM BB's laying around???
I assume you mean you have 68MM' laying around?

I would think if you had a 3rd hand, you could push in the BB from the NDS toward the DS.
IF you are astute enough to measure chain line, you should be able to figure out what "fudge factor" to use in your calculations.
Are these cup & cone, or cartridge BB's? Also note if your "test BB" is symmetrical.

You say you have a frame with 73MM and you also say you have 73MM BB's laying around???
I assume you mean you have 68MM' laying around?

I would think if you had a 3rd hand, you could push in the BB from the NDS toward the DS.
IF you are astute enough to measure chain line, you should be able to figure out what "fudge factor" to use in your calculations.
Are these cup & cone, or cartridge BB's? Also note if your "test BB" is symmetrical.



Oops. You are right. I edited my post. I actually have some old cartridge 68mm BBs lying around. And I have an old mtb that has a 73mm bb shell.

I have some flangeless non-drive (left) side cups, but why does that matter? The drive-side (right) cups have a flange on them. Is that going to be a problem?

I have some flangeless non-drive (left) side cups, but why does that matter? The drive-side (right) cups have a flange on them. Is that going to be a problem?
The flange on the drive side won't be a problem as long as the NDS cup doesn't have one. You will have to thread the NDS cup in well beyond flush to compensate for the wider bb shell so it can't be flanged. Just be sure the NDS end of the spindle is long enough so the crank arm clears the bb shell and chainstays.

Oops. You are right. I edited my post. I actually have some old cartridge 68mm BBs lying around. And I have an old mtb that has a 73mm bb shell.

I have some flangeless non-drive (left) side cups, but why does that matter? The drive-side (right) cups have a flange on them. Is that going to be a problem?


If all your doing is threading it in to do chainline measurements, you don't need to do anything with the non drive side, correct?

So go ahead and do it. Take your measurements and remember to adjust them by 2.5mm

If all your doing is threading it in to do chainline measurements, you don't need to do anything with the non drive side, correct?

So go ahead and do it. Take your measurements and remember to adjust them by 2.5mm


Alright. Thanks for the tip, but why do I need to adjust my chainline measurements by 2.5mm? I put in the old 68mm bb into the frame's 73mm bb shell. I got lucky. My 68x115mm bottom bracket that I put in resulted in the PERFECT chainline. So, I think I should just order a 73x115mm bottom bracket to fit the frame's 73mm bb shell. Should I instead order a new 73mm bb that is 2.5mm greater or lesser than the 68x115?