Need help with installing a stem with a shim.
 

I bought a Mercier Kilo TT last month and it has a 1" steerer tube. I bought a 1 1/8" bmx stem and a 1 1/8 shim so that it would fit. I went into it thinking it would be a simple task, but I was wrong. I can't get the stem to stay in place. It doesn't matter how much I tighten it. How far should the shim be inside of the stem? Any help or tips would be much appreciated. I'm pretty new at taking apart/reassembling bikes. Thanks!

If you have a 1 inch to 1 1/8th shim it should fit snugly between the untightened stem (a 1 1/8) and exposed fork, no matter where it is centered underneath the stem. Ideally your shim should be around the identical length to your stem, but this doesn't matter much. If this is not happening, my best guess is that you have some funky shim or stem, which components are they? I have a thomson shim with a road stems and it works fine, maybe its your BMX stem?

And in case you are doing something wrong with the installation, I have the same setup and this is how I replace/adjust my stem. First you wanna slide in your spacers, followed by your stem with shim underneath, and top it off with the top cap with at least 2mm between the stem and edge of the fork. Once your stem is centered, tighten your top cap snugly compressing the headset/spacers/stem (important step, or else you will feel play in your fork). Then tighten your stem firmly, which really seals the deal, and finally I like to undo the top cap a little so its not under too much stress (Im paranoid about damaging the locknut). If you are doing all this correctly, then my best guess is that your shim and stem somehow are different measurements.

Sounds to me like the shim's not long enough for the steerer clamp. Perhaps also insufficient thickness.
I'd try another.

I bought a Primo Casket bmx stem and a no-name shim. The shim is longer than the stem. The main reason for buying this stem was because I want a short reach. But now that I bought it (and bars to go with it) I'd really like for it to work out.

I've properly tightened everything a couple times, removing and adding some spacers to see if anything would work. No luck. Are there shims for bmx stems out there?

A shim longer than the stem won't work unless it's a flanged type, then maybe.
We need to see photos.
Here's some technical help.

If it's flanged then the flange sticking out is fine. Otherwise, if the shim is longer than the stem you'll have to cut it down, get another, or maybe use a 1 1/8" spacer to take up the extra shim length.

If the stem is the last thing on the stack then a 1 1/8 stem might need a 1 1/8 top cap or else put a 1" spacer above the stem then the 1" top cap. The spacer or top cap should push down directly on the stem, not the shim.

If the stem is rotating on the steerer tube after the clamp bolts are tightened, then you've got the wrong shim (which would be strange). You might be able to bring it back and exchange it but for what I'm not sure, maybe a different brand shim, your best luck may just be to bite the bullet and let a shop worry about it.

I'll never understand why they make bikes with threadless 1" headsets. If it's going to be threadless why not just go to 1 1/8".

Stem purchased:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT

Shim purchased:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT

You are lining the slot in the shim up with the slot in the stem right?

Those two things should work fine together. Are you still having trouble?

Yes, still having problems. And yes, I am lining the slot of the shim up with the slot in the stem. The shim is longer than the stem though.

Checking back...
Options:
1 Cut the shim down. It needs to be 1mm shorter than the stem.
2 Buy new stem.

Threadless headsets were invented before 1-1/8" steer tube diameters became industry standard.