The mountain bike crankset, or chainset, or crank is the component of a mountain bike which converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain. The chain in turn drives the rear wheel. It consists of one or more chainrings or chainwheel.

The crank arms of the bike can be damaged especially where they attach with the spindle of the bottom bracket. Or it might be damaged where the pedal threads at the distant end.

It is pretty easy to repair the crankset of a mountain bike. I will give you some instructions as to how to do that.

Usually due to a loose retaining bolt the damage at the spindle takes place. The crank arm slips a bit away thus it makes the connection loose. The spindle rotates a bit in the mounting hole and gradually, as time wears out, deforms it. When bad and crappy metal meets up with a robust biker slamming down on the pedal, damage is done at the pedal end of the crank arm.

At first remove the plastic protector cap if there is any. Then remove the retaining bolt which is usually the hex-head bolt. Then remove the bolt from the spindle. If the crank attachment is damaged, the crank might just fall off the spindle without any tries. Otherwise you'll need a crank extractor tool. Remember the position in which the crank was set for you will later need it as you place the new crank in its place. Never use a lever to take it off as you'll destroy the bearings in the bottom bracket if you do so! Thread the big end of the crank extractor into them, as far as it will go and turn the core of the crank extractor in. Now when you feel that the core has stopped turning easily you should know that it has hit the spindle. Now it is time to apply wrench to the core and then continue until the crank has come loose from the spindle. While inspecting the spindle for damage, check for side-to-side i.e. left to right motion of the spindle in the bottom bracket. Do listen for grinding or rubbing when you turn the spindle as if the bottom bracket is also damaged or is bad you ought to replace that as well.

In order to remove the chain-side crank, put the chain around the frame at the bottom bracket. Now it is time to use your hex wrench and crank extractor exactly as you did on the other side earlier. The crank will come off with the chain rings attached to it. To remove the chain rings from the crank, unscrew the locking. Now place the new crank exactly in the same position on the spindle as the old one. Apply Loctite to the threads of the retaining bolt. Tighten the bolt, and replace the protector cap.

You can get a picture guide to help you complete the task if you have a hard time understanding the instructions. But know that it is quite simple.