Are there Two sizes of 1" Headset?

[Jonny69]

There are two common sizes: 26.4mm is the most common and 27.0mm is the slightly less common JIS size.

 

[Jonny69]

A bit of an update.

I reached for the cheap guestimator again and got these measurements:

Inner Diameter of the bottom headset race that goes on the forks = 26.30mm
That is a Richey Logic I just found in the loft.

Diameter of the metal it is supposed to go on = 26.80mm

The guestimator is probably not 100% accurate; it is a Clarke after all! I have measured a few times though. Half a poxy mm! That is only 0.25mm per side.

I have checked the threads and they seem to match. I just need to make it fit. I think I am going to file/sand the headset piece until it interference fits the forks. Before I do that, I'm going to make sure the other bits fit in the frame.

I think you’ll need to use a better vernier to know for sure, but it does sound like your fork wants a 27mm crown race and the Ritchey Logic is a 26.4mm. I think the fork should measure bob on 26.4 or 27.0 and crown race should measure up a little under, so maybe 26.35 and 26.95 but I can’t remember my press fits.

If the vernier is fine and the fork is in fact measuring up to 26.8mm, a 27mm crown race will just rattle around and you’ll need to put a strip of 0.1mm shim all the way round when you fit it. Coke can shim is about 0.15mm unfortunately so I don’t know if you’ll get that in even though it’s soft aluminium.