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

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bellweatherman
05-27-08, 12:13 AM
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.

Soil_Sampler
05-27-08, 12:22 AM
if the bb cups are flangeless, yes

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

Bill Kapaun
05-27-08, 02:04 AM
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.

bellweatherman
05-27-08, 11:36 AM
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?

HillRider
05-27-08, 04:29 PM
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.

dobber
05-27-08, 04:40 PM
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?