Most modern mountain bikes use a 1 1/8" steer tube that is smooth. The stem clamps to the steer tube to keep everything held together and the headset bearings in proper tension. The Diamondback, although 1 1/8" uses the older-style threaded steer tube, where a large nut threads down an hols everything together and tensioning the headset bearings. The stem then slips down inside the steer tube and a bolt pulls a wedge up into the bottom of the stem, essentially jamming the stem in position. You can upgrade to a 'threadless' stem but it's generally not advised because the stem can't get enough bite on the threaded portion it would need to clamp to on the existing fork.