M900 XTR Headset Crown Race - slack on the fork!? How?

[rjsdavis]

Hello to all

I've been upgrading my Rossin from Deore XT to XTR, and the only real groupset issue I've got, is that the crown race of the 1" headset is slack (indeed, pretty damn loose) on the cone of the fork.

The Deore XT headset that was removed was a good, tight and snug fit - including the crown race. Therefore, when the crown race of the XTR headset that I picked up from Racer X was slack?! I've attached some pics below, of the movement that can be made pushing around the cone of the fork. I just don't understand this. Although, now I'm thinking about it, could it be that the crown race of the headset is 1 1/8th in diameter and the rest of the headset 1"? I don't know...

I can't see any damage to the crown race - I can't see any wear to the crown race, therefore, I cannot understand why it's slack. I've added some pics below, and would appreciate anyone's thoughts as to what on earth is going on here!

 

[one-eyed_jim]

There were two common 1" threaded headset standards: JIS and ISO. Your Rossin must be ISO (or the near-identical Italian standard) and your new XTR headset must be JIS.

http://www.sheldonbrown.com/headsets.html

The major difference is a 27.0mm (JIS) versus 26.4mm (ISO) crown race seat, but there's also a small difference in the press-fit dimension of the cups.

 

[rjsdavis]

Thanks Jim - I suspect that this is exactly what the problem is...

Feck. That's really annoying. What are the chances of being sent a JIS headset? It didn't even occur to me to check at the time. Although, having said that, the gap (as you can see in the pics) looks like it's far bigger than just 0.5 of a mm!

 

[rjsdavis]

You were exactly right Jim - a NOS XTR headset from Kingroon, has just been fitted and completely cured the problem - I can't believe that I didn't recall that this was even a possibility from my industry days!

 

[one-eyed_jim]

Happy to help.

It's one of those things that's obvious if you know what to look for, but puzzling if you haven't seen it before. And the gap always looks bigger than it really is!

A good Vernier caliper is one of the best investments a bike-tinkerer can make.