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I just got a new Demolition Stem.I tight it with all my force And It keeps slipping side to side Like it doesnt match with the fork lined up And whenever I check it its fine but after a while it is always not lined up with the fork..Please help me

Demolition Stem

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Make a small mark on the steerer tube when the stem is aligned. The next time the stem moves verify the stem is moving in relation to steerer. It is possible that the steerer is moving in the fork crown. – mikes Aug 8 '13 at 2:05
Use a vernier caliper to measure the diameter of your stem and the head tube. If they aren't compatible, you may break the thread on the stem and still not be able to hold the fork in proper place. – Akshay Aug 8 '13 at 22:46
-1 for asking a good question and then walking away and never coming back. – Carey Gregory Aug 18 '13 at 5:55

Is it possible you tightened the pinch bolts before tightening the bolt that runs vertically and holds the top cap down? If you do that, the entire assembly won't be fixed tightly on the vertical and will allow side-to-side play. I did that once and the symptoms were exactly as you describe. To check this, loosen the pinch bolts and the bolt in the top cap. Now tighten the top cap bolt first, then the pinch bolts.
Be very carefully while tightening the top cap. If tightened to much, it can overload and ruin the headset bearings.

Here's an image from Sheldon's site showing the bolts I mean:

enter image description here

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Also, examine the space between the "fingers" that the pinch bolts pull together. If there isn't enough clearance to slip a thin sheet of paper into the crack (with the bolts tight) then the clamp is the wrong size or you need a shim. – Daniel R Hicks Aug 8 '13 at 15:09
+1 for pictures – MrBoJangles Aug 8 '13 at 15:34
I don't think what you describe is the OP's problem. – cherouvim Aug 8 '13 at 16:21
@cherouvim - Maybe not. What do you think the problem might be? Or at least why don't you think what I suggested is the problem? I can see from your profile that you know how to answer questions. ;-) – Carey Gregory Aug 9 '13 at 4:24
I think that the OP's problem is that the stem gets missaligned to the fork. What you describe is play on the forks due to poor installation. – cherouvim Aug 9 '13 at 6:35

One of the most common reasons for this in a new BMX bike is that the riders tighten the fork bolt without loosening the stem bolts. I used to do the same thing when I first started riding and I just helped 2 other people in the past 3 weeks with the same issue. Once you understand how the fork/stem system work it makes more sense. Back in the 80s, all the BMX necks tightened from inside the forks using a wedge, when they moved to the more modern necks, people still tried to tighten the forks using the bolt that is in the same location (I did too).

The best way to tighten your neck/forks is to loosen the stem bolts (A) then get the fork bolt (B) 'sung', do not tighten hardly at all or you will lock up the headset. Move the forks back and forth to ensure that they are not too tight, not too loose, but JUST RIGHT! Then go back and tighten the stem bolts (these you can crank on if you like, but they usually recommend less than 100 lbs)

enter image description here

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When you tighten the pinch bolts of a stem, you should notice how the gap around the steerer closes. If it closes so much as the two sides touch, it is more likely that any further tightening will only press one side against the other, not around the steerer, which will give the described symptom. This observation should also be made when tightening the handlebar clamp.

However, this situation will only arise in case of a size mismatch, on in a really severe case of worn components. The latter will usually be detectable by simple inspection, but is less likely.

Another cause known to me for the described symptom is that there where "dust" trapped between the stem and the steerer. This dust is wear byproduct, it is actually metallic. I believe that this dust is ball-bearing like, so it allows pieces to slip even if properly tightened. The solution here is to clean the components prior to assembly. (This effect actually happened to me with an aluminium frame and a titanium seatpost. The seatpost was wearing down the seat tube, generating this "dust". In such situation the seatpost would lower itself by just riding the bike. The solution was to exchange the seatpost for one with a smoother surface)

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