First of all... does your dirt bike NEED to have the steering head bearing adjusted? Those of you who are seasoned dirt bike trail riders or motocross competitors will already know how to check. Newer or infrequent riders may not even know what I'm talking about. So here's how to check your dirt bike's triple clamp or steering head bearing.
Raise your dirt bike so that the front wheel is completely off the ground, but in an upright position. Stand, kneel or, if it's been a long day, sit in front of the dirt bike and grab the bottom of the forks. Pull gently back and forth. If the wheel assembly moves front to back, it's definitely time to tighten things up. If your dirt bike's steering head bearing is this loose it will (and hopefully already hasn't) wear that nice circular hole into an oblong one.
Your steering head bearing may not be as loose as above, but still need some tightening. The second test you'll want to do is, again, with your front wheel off the ground. Turn the handlebars all the way to one side and flick the handle bars. You should only get one bounce, maybe two. Also, the handlebars should be easy to move from side to side, but shouldn't flop over to one side when you let go.
Following is the quick and easy way to adjust the tension on your steering head bearing. You may have to remove a couple of things to get to the parts, but many dirt bikes have everything exposed and accessible, so you'll be done in no time. There may be some slight variation between different models of dirt bikes, but the basics are the same.
Once again, start with the front wheel off the ground. You absolutely cannot do this with the dirt bike resting on the front wheel.
If you have to remove any parts – for example, the handlebar – mark them before removal so you can put everything back exactly where you had it and want it.
Loosen the fork clamp bolts – also referred to as pinch bolts – so that the fork tubes can move independently of the steering stem.
Now loosen the steering head nut, which is sometimes called the crown nut. The steering head nut is labeled as #1 in the diagram. You probably don't have to remove it completely.
You're now ready to tighten the adjusting nut(s) – there's usually one, but sometimes a second one as a lock nut. The adjusting nut is labeled as #2 in the diagram. You'll notice the adjusting nut is basically a ring with notches in it – also called a ring nut. A special wrench is available for the adjusting nut. However, I personally never have to adjust this very often, so I'm comfortable just setting a screwdriver in one of the notches and gently tapping the handle of the screwdriver with a mallet or hammer. The choice is yours.
Make slight adjustments, tightening a little at a time, and then check the play in the steering. Most manuals call for you to eliminate all play when you grab the bottom of the forks and rock them back and forth, but not so tight that it binds or sticks when you turn the steering side to side.
Once you have the bearing adjusted properly, re-tighten the head nut. Double check for play and binding. Then re-tighten the fork clamp bolts. Torque everything to the specs in your dirt bike's manual.
Perform the steering head bearing checks mentioned above after you've done some more riding on your dirt bike, and then check periodically after that.