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I replaced an old Raceface Deus stem with a nice Thomson one and I forgot to refit the slim spacer that fits underneath the stem. As a result the spacer has now gone 'missing' and I've tried tightening the stem/headset up without it but it has a small amount of play in the fork which obviously is no good. The fork steerer sits less than 1mm (almost level)from the top of the Stem and I assume this will be increased and the steerer tube will sit lower once i start to tighten the top bolt and compress the headset. So my question is this.... How thick a spacer will i need to get it all to tighten up properly? I think the 'missing' one was about 5mm? The stack height on the old R/face stem and the new Thomson one look identical so I assume i just need a 5mm spacer?
Sorry if it too vague a question. I can just buy a pack of spacers and play with them but wanted to know if i'm going about it in the right way or if there are other factors that I should check first?
T.I.A
Yes, that's the one. My only concern is the Thomson stem bolts, one is quite close to the top and if I stack to big a spacer underneath then it might make the stem unusable. That's why I was wondering how much the headset compresses once it's tightened properly.
You are going about it the correct way.
From what you describe I would have thought 2 x 5 mm would cover it.
A quick fix would be to use one off another bike and place it on top of the stem.
Refit top cap and tighten to the correct load to remove play in the headset without getting any binding.
Tighten stem bolt.
Remove top cap and spacer.
Measure from the top of the stem to the top of the steerer tube.
Add 2-3 mm to this measurement to get the required spacer depth.
Replace the top cap and then you can ride the bike until your new spacers turn up 🙂
Thanks Jes. I shall try that before I go spending. Tell me though, why does the top bolt not compress the headset enough so I don't need to use spacers? It seems a fairly sturdy bolt but am I missing the point completely and the spacers play an important part of the clamping process other than just adjusting the height.
Yes your correct, the steerer needs to be 2-3 below the headset, head tube, stem and any spacers stack.
The top cap rests on the stem/spacer washer and the cap bolt fits into the star washer (or other devise) in the steerer tube.
When the bolt is tightened it pull the steerer up or compresses the stem, headset etc down.
If the steerer is longer than the rest of the stack the top cap just bottoms out against the top of the steerer tube so you are unable to take out the play in he headset.
Hopefully that makes sense 🙂
That makes perfect sense. Thanks again Jes.
This is the problem i have.
Stem with no spacer and top cap tightened up [IMG] 
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Stem with no spacer... [IMG] 
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Stem with added 10mm spacer underneath (borrowed from another bike) [IMG] 
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Stem bolt showing above the fork steerer with 10mm spacer added [IMG] 
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So i'm guessing a 5mm or 3mm spacer will be the correct way to go?
Excellent photos, I can see why this was a bit perplexing.
Womble from the second photo I would have thought that would have been perfect, I use the Hope head doctor system on my setups.
I can only think that the top cap has a second and quite deep shoulder that rests on the top of the steerer tube.
3-5 mm looks good to me, although I would still set it up will the washer on top of the stem, them measure from top of the stem to the bottom of the outer shoulder of the top cap.
A flatter top cap might be a better option. I certainly wouldn't want to put a 5mm under that - 3mm maybe and it might work on top. Tight one though.
put the stem on the steerer - the spacer on top - do the pre-loading of teh top cap - do up the stem bolts - remove top cap and spacer - put top cap back on.
the spacer will sort out the preloading of bearings, once the stem is done up the top cap is redundant.
As a temp fix you could put the 10mm spacer on top of the stem under the top cap, if the preload bolt is long enough.
Ok, so I've used the 10mm space on top and have wound it down. It has made a slight improvement and the steerer is now flush with the top of the stem. This is how it looks with no spacers [IMG] 
[/IMG]
Bought a 5mm spacer from the LBS (smallest they had in stock) and tried it again and this time i managed to clamp the forks up nice and there is no movement... Happy days. It's just the top stem bolt that worries me now. [IMG] 
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With the 5mm spacer in place and the stem bolts nipped up and the steerer/forks nice and solid, the top of the steerer tube now sits half way on the top bolt (where the white lettering is on the stem). Do I put the spacer on top, go for a 3mm spacer to improve clamping safety and a neater look or is this set up ok?
yup, look at the bottom of your top cap and it will become clear.
I would get a very slim spacer - 2 or 3mm or so and either put it either under the stem or above it. For now do as above and put the spacer you have above the stem, set the preload on the headset and clamp up the stem and then remove the spacer.
edit: just saw your new post.
Swap the new 5mm spacer to above the stem and leave it there. You won't really notice the spacer on top.
Thanks Andy and everyone else that has contributed. This really should have been an easy fix if I hadn't mislaid the original spacer 🙂 I'm new to bike maintenance and its little issues like this that have me scratching my head.... But I'm learning from these mistakes 🙂
Agree just drop the spacer on top of the stem.
Hey learning is the fun part, your a natural 😀
You could always trim a couple of mm off the steerer. 😉