I have a 80's bike (Limongi) whose fork diameter where the crown race would sit is 26.65 - 26.67mm according to my calipers (I measured carefully in a few spots). LBS told me that this was Italian standard and I bought a Campy Record headset for them to install. The crown race broke. The LBS said they'd get back to me with a solution next week.

In the meantime I'm losing trust that they'll do the right (safe) thing. I don't want whatever crown race they find and install to snap while doing a 70 km/h decent.

Do any pro mechanics have any advice?

Your mechanic was asleep at the switch. The ISO (Campy) crown race nominal dimension is 26.4, not 26.6. The actual size is 26.42-26.45 (.05mm oversize for interference). If you measured correctly the .2mm difference is too much and will often split crown races which are too brittle to expand that much. The right solution is to have the crown seat milled to spec. A job that any decent road shop should be able to do for $25.00 or less.

While you're at it, have someone confirm that the head tube dimensions are also to spec, though head tubes are relatively ductile and more forgiving of an overly tight fit.

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.

Thanks for the advice although I got the headset parts onto the headtube by hammering them in (with a wooden block). They went into relatively easily.

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.

Thanks.

Makes sense, my last 20 yo race (Campy Croce d'Aune or maybe it was Chorus) lasted many 1000s of kms but I wanted to replace it because it was starting to become pitted (I was lazy and didn't repack as often as I should have).[/QUOTE]

Yes. It needs to be just slightly larger than the ID of the crown race to make an effective interference fit. Campagnolo made a Go/No-go gauge to quickly assess the race seat:



Crown races crack when they're driven on crooked; some seem less tolerant of misalignment than others. We broke dozens of Super Record crown races at Trek, but I don't recall breaking a single Record crown race.