tolerance on BB size

[GazK]

I bought a Raleigh gran sport frame and forks, which was originally fitted with a gran sport BB. When I got it it was fitted with a cheap FAG ISO BB, with an axle length of 119mm. To this I fitted a 7400 dura ace chainset, and the resulting chain line is about perfect. But I'd like to fit a better quality BB, preferably campy. But all of the ones I see on eBay and such seem to be 112mm axle. Is all of the difference between 120 and 112 seen on the drive side? Or is it split 50 50? There is about 10mm of clearance between stay and chain ring, but I don't want to mess up a chain line that works well. Did campy do any 120mm Square taper BBs?

 

[hamster]

You shouldn't be running a JIS chain set on an ISO BB. The end is slimmer and there is a danger that the crank cannot be tightened fully.
Campag BB sizes are limited as they only made them to fit their own chain sets.

 

[Ippikin]

I bought a Raleigh gran sport frame and forks, which was originally fitted with a gran sport BB. When I got it it was fitted with a cheap FAG ISO BB, with an axle length of 119mm. To this I fitted a 7400 dura ace chainset, and the resulting chain line is about perfect. But I'd like to fit a better quality BB, preferably campy. But all of the ones I see on eBay and such seem to be 112mm axle. Is all of the difference between 120 and 112 seen on the drive side? Or is it split 50 50? There is about 10mm of clearance between stay and chain ring, but I don't want to mess up a chain line that works well. Did campy do any 120mm Square taper BBs?

Yes agreed Campag & Shimano have different tapers. Am surprised it did not work loose or creak like crazy, 10mm is a biggish gap (my bike with Campag double 39T is 5mm or so, what size ring are you running and where are looking to assess the chain-line?

 

[GazK]

I checked with Sheldon before I did it, and his advice is that its not generally a problem. I'll try and get a pic of the back side of the chainset to see if there is a gap to the end of the taper... watch this space. Chainrings are 42/52, chain line is eyeballed from the back of the bike when in middle rear sprocket.

 

[GazK]

OK here we go - I thought I had checked for this exact issue when I fitted the chainset, and sure enough there's taper to spare:

And here is the chainline, 4th sprocket of 6 onto inner front:

Returning to my original question, would a longer axle be spread 50/50 or all on the drive side?

 

[ScillySuffolk]

Dura-Ace 7400 requires a 112mm BB anyway, so no problem.

EDIT
To answer your original question: a modern BB will be 50/50.

 

[Midlife]

Longer axles usually have the extra length biased towards the drive side........

Shaun

 

[hamster]

The issue on taper is that the ISO one is slimmer. So the crank goes fully on, then the bolt tightens fully onto the end of the shaft and bottoms out...but the crank is not fully tight on the taper. In the end the flats get mangled. Up to you.