These are cheap but important to have afew sets in your pit bag. They are the original size of the ones that come with the MTX4R. These are made of plastic but because it spins at really high speed and pulls the belt, they tend to wear down the teeths on the pulleys. If your thinking of getting alum pulleys, let me tell you that they in my expierence wears out just as fast if not even faster for some apparent reason, do i need to tell you that they cost alot to change 5 or 6 alum pulleys if your counting the rear diff ones also. So have these handy for changing, they are inexspensive. The other items to have are replacement belts.