whats the o-ring type thing at the bottom of a fork steerer that the headset rest on?

I took it off my sids and used them on my need fork – but now I’m going to use the sids on another bike

Crown race?

Are on about the metal thing that you have to hammer onto the fork steerer and use a screwdriver adn a hamemr to get off or a plastic/rubber o-ring type thing? If it’s the former then it’s the crown race. If it’s the latter then it’s just sealing for the headset.

crown race – cheers guys

where can I buy one to fit a 100mm sid race?

They’re specific to the headset, not the forks.

You’ll need to get one that’s specific to the headset, not the forks.

EDIT- Beaten by 30 seconds! lol

ordered this headset http://www.merlincycles.co.uk/Bike+Shop/MTB++Parts/MTB+Accessories/Headsets/FSA+Orbit+XL2+Headset_369.htm

sounds like its got one