Headset cup / steerer tube diameter?

[foz]

I thought all ISO and french headsets used a standard 30.2mm headtube/cup diameter? This afternoon I was tinkering, as you do, and realised that a dura ace 7400 french threaded headset that I have actually has 30.4mm cups. And I can fit an ultegra 6400 headset with a 30.2mm cup (measured by me) by hand into the frame it came out of.

Now I was hoping to use the dura ace on my nortelle (if I ever get round to sending it to the painters), but I don't fancy pushing the headset in if it's too tight...

Can anyone shed any light on this? Is my dura ace headset just lightly larger than normal? Or did they make them in that size? Am I likely to cause problems if I squeeze it into my frame? How easy would it be to reduce the cup size slightly without a lathe?

Thanks!

 

[Midlife]

Strictly you are right but in practice it never works out 100%

Having fitted quite a number of headsets in my time some are tight, some are firm and some are real nightmare........the worst was an expensive EDCO competition alloy that I fitted to my Bob Jackson.....the lower cup was so tight it flaked off some paint but the upper was loose and I could press it in by hand

The headsets are basically machined the correct size, it's the frames that are different....brazing, filing, painting, laquer, more filing........ I'm amazed anything fits !

Shaun

 

[foz]

Thanks Shaun! I know nothing is ever the size it's supposed to be... This headset is definitely 30.4mm all the way around, which made me wonder and check sheldon for odd sizes, but I've found nothing mentioned :? Makes me wonder if the frame is oversized from having the headset squeezed into it, or if someone checked all the headsets to find a larger than usual one to fit the frame! Either way, I don't think I'll be using it, which is a shame as it's very nice condition and would work well with the build! I don't want to risk the frame, trying to squeeze it in there. Maybe some light filing and sanding would bring it down to 30.2-ish? Ever done that to a headset? Is it likely to work, or am I more likely to stuff it up beyond repair?

thanks, Andy

 

[Midlife]

That's odd....all headsets came as a standard size which was 30.2mm, even Raleigh.

Bottom brackets were different, we could re-tap to a larger diameter and the BB sets came with a big O stamped on them but that's another story.

As we used to make bike frames in the shop we had a range of tools to ream out the head tube after painting etc but they were always one size.

Is it a chromed headset?

Shaun