Because of the nature of freestyle BMX as a sport, broken bike parts are a common occurrence and repairs to BMX bikes are frequent. One of the most frequently broken parts of a BMX bike is the sprocket, which is easily dented and bent while performing chain-ring grinds and other tricks. Rather than shelling out the cash for a new sprocket when yours gets bent, learn how to repair your sprocket at home and get back out riding quickly.
Attach a 15mm wrench to the pedal spindle of the pedal on the non-drive side crank arm of your bike. While holding the wrench steady, spin the cranks forward (counterclockwise from the non-drive side) to loosen the pedal spindle. Twist the spindle clockwise, as it is reverse threaded, to remove the pedal from the crank arm.
Attach an adjustable wrench to the outer bottom bracket nut at the bottom of the non-drive side crank arm. Spin the crank arm clockwise while holding the wrench steady to loosen the bottom bracket nut. Loosen the nut completely, then remove it and the spacer beneath it from the bottom bracket. Push the crank through the bottom bracket to separate the crank from the bike.
Pull the sprocket off the drive side crank arm and separate it from the rest of the crank by sliding it over the non-drive side crank arm. Locate the bent portion of the sprocket and set this portion tightly in a vice or clamp. Slowly and carefully push or pull on the sprocket to straighten out the bent portion. It may be wise to sport a pair of gloves while doing so to reduce the risk of cuts and scratches.
Remove the sprocket from the vice and slide it back into place on the crank. Slide the crank back into the bottom bracket, non-drive side first, making certain that the drive-side crank arm sits flush against the bearings inside. Slide the bottom bracket nut and spacer back onto the non-drive side crank arm and use an adjustable wrench to tighten them.
Hold the pedal spindle of the non-drive side pedal in the pedal threads of the non-drive side crank arm. Spin the cranks backward (clockwise) to thread the pedal onto the crank arm. Use a 15mm wrench to tighten the pedal spindle counterclockwise so it won't come loose while you're riding.