BMX stem slipping; how to stop? 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
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.