Headset spacer -- where does it go?

[erbfarm]

I just followed the tut on how to overhaul your headset except I didn't take the headset apart and remember where everything went. Well, now I have it back together and it seems to work ok (moves freely, no play) but I've got a small spacer leftover and can't figure out where it goes. It doesn't seem to fit anywhere. It's small. Is it a big deal if I leave it out?

[Alex Ramon]

It's probably important... are you able to upload a photo of the headset and spacer?

[Santa Cruz Bullit]

Sure it's not a washer from your stem bolts?

[DaveM]

If you have a threaded fork, the spacer probably goes between the two upper headset parts (cup and locknut). As long as the top locknut is not bottoming out against the top of the fork's steerer tube, the spacer is not critical. However, it does help you tighten the locknut withour turning the top cup and messing up the adjustment on the bearing. Also helps prevent the headset loosening up during riding. I'd put it back in, but it won't hurt anything if you've been riding without it.
One other thought. Some headsets have little dust cups that fit intot he cups to fill the gap that lets dust in. These are a bit larger diameter than spacers and are not smooth and flat on top an bottom. Also not critical, but they will keep teh heaset much cleaner.