Originally Posted by
e2rider
Thank you FBinNY! So the crown seat should be milled to between 26.42 and 26.45 in order to produce the interference fit with a 26.4 ID race? Or should it be milled to 26.4mm? I ask because I have the cracked race (it didn't split - the flange just broke off) and its ID seems to be 26.37mm. I'll try to find the nearest frame builder 'cause I have yet to find a shop in my city where the mechanics seem competent.
26.4 is the nominal, and the crown races themselves are dimensioned to that -0.00 +0.02mm. The cutters are made to cut 26.45 ±0.02mm so that provides for the interference. There's some latitude in the amount of interference, but there has to be some. However .2mm is more than a heat treated bearing race can accept.
Note also that modern crown races are not as substantial is their predecessor, so an amount of interference beyond ideal that might be tolerated in the past will crack a more modern crown race.