[CR] Stronglight Bearings Question

(Example: Framebuilding:Brazing Technique)

Content-Class: urn:content-classes:message
Date: Tue, 15 Dec 2009 09:36:57 -0600
Thread-Topic: Stronglight Bearings Question
Thread-Index: Acp9nG+OjSxuYGowRn2nNoIleU0zPA==
From: "John Hurley" <JHurley@jdabrams.com>
To: <classicrendezvous@bikelist.org>
Subject: [CR] Stronglight Bearings Question

Michael suggested the rattling ball bearings in Ben's bottom bracket might be due to poorly aligned bearing cups:

"On a production bike it is almost a certainty that the BB shell was not faced and/or tapped. Because brazing heat causes the BB shell to expand, and then unevenly contract after cooling, the BB cups might be / probably are slightly skewwed to each other and this might be causing the bearing noise you hear."

"The noise you hear might be lessened or eliminated altogther after machining the BB with a good quality PILOTED facer and taps."

This explanation doesn't seem to agree with what little I know about bottom brackets, so I'm looking for insight. First, I thought all bottom brackets were threaded to receive the cups, so I can't understand bikes leaving the factory without having been tapped. Second, the orientation of the cups is determined by the threads and nothing else. If the threads are straight, the cups will be straight. If the threads are skewed or misaligned, the cups will be off no matter what you do with facing. Hopefully the curved surface of the cup affords a certain tolerance for error in the cup's alignment, so the balls can find their own correct line even if the cup is a little crooked.

I have heard of the importance of facing the bottom bracket, but as I've said, I don't see how it would have much effect on the alignment of the cups. I would think the importance of facing would be in helping prevent the lock ring or the fixed cup from working loose, by providing a nice flat surface to tighten against. If the facing was poorly done, I could see it being improved by re-facing.

However, if the bottom bracket threads were not cut correctly, how could this be repaired? Just retapping would make a mess wouldn't it? Would you first re-fill the old threads with brazing material and then re-tap? Can you re-tap oversized? Replace the entire bottom bracket?

Could Ben's clicking noise be the result of one or more balls having escaped into the interior of the shell during assembly so he doesn't have the full eleven in one side?

John Hurley
Austin, Texas, USA