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tolerances are 41.96 +0.025/-0mm (meaning the bore can be slightly oversized but not undersized)
Well, that's an odd way of saying it. Normal engineers would call that 42mm -0.015 -0.040. Yeah, that's right, I gave negative values to both tolerance limits. It's described that way because the tolerances are actually interference fits on a nominal 42mm diameter insert. The bearing itself will be 42mm +0.000 -0.011 for a standard industrial quality cartridge, so the BB30 standard means that even the smallest bearing won't rattle around in the biggest shell, and the biggest bearing won't either split open the smallest shell or get squeezed too tight to run freely by the 40μm interference.
gbj_tester
"Cannondale's Chris Peck provided BikeRadarwith specific details. While BB30's bearing bore tolerances are 41.96 +0.025/-0mm (meaning the bore can be slightly oversized but not undersized), PF30's requirements are 46.00 +0/-0.05mm – exactly half as rigorous."
http://www.bikeradar.com/au/gear/article/complete-guide-to-bottom-brackets-36660/