Originally Posted by
Lovenutz
26.4 vs 27.0 crown races on 1" threaded forks. Are they interchangeable between headsets?
The new fork I bought needs a JIS crown race, but the headset I plan on buying is supplied with an ISO. I don't have anything now to test.
I was under the impression that after the correct size is seated on the fork, that they could be used interchangeably. My LBS is telling me otherwise.
Any experience?
I'm not exactly sure what you're asking.
But let's start with nomenclature. ISO does not specify a crown race diameter. It is a thread form standard and that's about it. The 27.0mm is in fact the JIS spec, but the 26.4 is what was previously the Campagnolo standard, but nowadays more often called either the Professional or the "Modern European" standard.
Now as to interchageability: If your crown race seat is milled to 27.0, you can have it milled to the Professional standard if that's what you want to use. But once your fork's crown race seat has been milled to the Professional standard, there's no way a JIS crown race will ever fit again. You can't cut it bigger.
The other thing to know about the Professional and JIS standards is that they specify different cup skirt ODs. Press-fit cups are interference fit. Professional spec is 30.2mm, JIS is 30.0mm. The ID of the head tube bore has to be 0.1mm less than that for the interference fit. So a head tube milled to Professional spec won't hold JIS cups tightly; they'll slip in and out by hand, in fact. A head tube milled to JIS spec will make for a too-tight fit of Professional cups, and it might actually stretch to accommodate them (seen it.)