A bicycle fork serves two indispensable functions: as a point of attachment for the front wheel and as a platform for the steering assembly (handlebars, stem and headset). BMX forks are no different. In fact, many have the added responsibility of playing a crucial role in helping a rider perform certain stunts and tricks. However you choose to ride your BMX bike, assembling the fork properly at the outset will help ensure safe and effective operation of your bike.
Lay out the parts of the headset. Essentially the headset attaches the fork to the frame of the bike. A usual headset includes two races, two bearing cartridges, one dust cap, one top cap and a compression bolt.
Lower one of the fork races over the fork tube until it fits flush against the base of the fork. The race is a thin metal disk with a bevelled edge. The bevelled edge of the race should be face up.
Install one of the two bearing cartridges into the bottom of the bike's head tube. The cartridge contains a tapered edge, designed to mesh with the head tube. This tapered edge should be face up.
Pass the fork tube through the base of the bicycle head tube. Afterward check that the fork and head tube mesh properly. There should be no space between the two.
Place the second bearing cartridge over the fork tube. In this instance, the tapered edge of the cartridge should face down, toward the tube.
Install the second race, fitting it atop the upper bearing cartridge. When installing the race, the bevelled edge will be face down, toward the cartridge.
Fit the plastic dust cap over the upper race and bearing cartridge.
Lower the handlebar stem onto the fork tube so that its base rests on the dust cap. The top of the handlebar stem should cover the fork tube entirely. If it does not, remove the stem and place the necessary spacers onto the fork tube. These spacers (plastic or carbon disks of various widths) will bring your stem to the proper height.
Place the headset top cap onto the tip of the handlebar stem. Holding the handlebars straight, thread the head set compression bolt through the centre of the cap and use a proper size Allen wrench to tighten the bolt.
Tighten each of the Allen bolts on the side of the handlebar stem. As always, bolts will range in size, so fit your Allen wrench to the bolt carefully before tightening to make sure of the fit.
Check the headset adjustment by moving the handlebars back and forth. If you find that the steering binds (does not turn freely) loosen the bolts on the side of the stem and further tighten the compression bolt. After tightening the stem bolts, recheck your adjustment.