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I recently bought a BMX bike that currently has all stock parts. Recently, with the school and weather, I haven't been able to ride it that much. Anyways, when I picked it up from the shop, the bars were positioned leaning forward, not parallel with the forks, so I adjusted them.

After playing around on the bike a bit, I noticed that the bars would slip forward if I put a good bit of pressure on them, so I completely removed the bars. In the stem there was a lube, I guess for the screws, which I mostly wiped off because I figured that wasn't helping the slippage. I also tightened the bolts pretty tight, and I had to use a hex wrench + a cheater bar to get them pretty snug... I could go further, but I don't want to snap off a bolt in the stem and have to buy another.

So, does anyone have any suggestions on how to lower / stop the bars from slipping forward? I'd prefer to not cut grooves in the stem (unless it's the bottom, the bottom part of the bracket is completely smooth) but if that's what it would take I'll do it. Does anyone have any other suggestions?

Note: I did tighten the bolts in the correct pattern

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You should never use a cheater bar! Aluminum is not as resilient as the cap screws securing the stem plate and there is a real risk of shearing the threads out of your stem. The same is true of the other hardware on a bicycle. –  WTHarper Feb 28 '13 at 3:07
    
Indeed, if things are slipping in spite of correct bolt tension, it's some other problem. Maybe a shim is needed. –  Kaz Mar 1 '13 at 2:42
    
Did you check whether there was space in the gap? If the gap is pulled all the way tight you need a shim or different parts. –  Daniel R Hicks Mar 1 '13 at 3:43
    
No, the gap isn't fully closed - from reading online I think that the bar may be ovalized from where the shop initially tightened it up, but I will have to check tomorrow (I am at school at the moment).. I will be able to give some more detail then –  ekaj Mar 1 '13 at 4:58
    
Time to buy a torque wrench. Don't wait until you ruin a part that cost more than the torque wrench. Or lose a tooth worth more than the torque wrench. –  Blam Jun 17 at 20:55

4 Answers 4

You could try wiping the grease in the stem off with some alcohol or thinners. Even a little bit in there could be a problem.

I'd leave cheater bar alone and try and get them snug without using it. Stems aren't something you want to fail suddenly.

Have you checked the dimensions of the bar and stem?

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If the parts move in spite of the tightening (with extra torque via the cheater and all), that shows there is sizing issue: the part is fully closed, yet the handlebar can still rotate.

You can use metal from a pop/beer can as a shim between the stem and the bar.

Also, perhaps the stem's clamp could be machined to shave a fraction of a millimeter from its mating surface, reducing the clearance in the handlebar channel. You will, of course, be losing whatever powdercoat finish is on that inside surface, though. A similar thing is sometimes done with old engine heads to improve compression.

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Clean the stem / bars, use a pop can shim, maybe put some blue loctite on the bolts to prevent loosening. Don't use a cheater bar on aluminum. –  Benzo Mar 1 '13 at 14:54
    
Don't use a cheater bar on anything, unless it is required to produce the specified Nm or lbft of torque. –  Kaz Mar 1 '13 at 19:20

Try sanding the stem and bars because most BMX company's paint there bars and stems all the way so there is no grip. If you sand the inside of the stem and the grippy part of your handlebars it should be fine.

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I had this issue once so I cut an old tube (about 4" of it) then wrapped my bars with it and tightened the bolts tight in the correct pattern and my problem was solved? A rubber shim I guess... By using a tool for tighter leverage this may have ovaled the bars on it's own?

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