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"Bicycle wheel bearings" should last for years and provide you with thousands of miles of travel---but they won't last forever. In some cases they even fail completely if they haven't been maintained correctly. Part of a maintenance schedule should include replacement of wheel bearings before they fail. The procedure isn't that difficult if you have about an hour and a few small tools. Front and back wheels are similar in procedure to change bearings and only require slightly different tooling.

You must get yourself some flat open wrenches, Park Tools makes good ones, make sure they are in sizes 13mm and 15mm. Those are the most common sizes. You won't get away using anything else for the bearing cones, so don't even try. Any kind of adjustable wrench will do for the locking nut.

Put the flat wrench on the bearing cone between the hub and the locking nut. Put the adjustable wrench on the locking nut. Hold the bearing cone in place and unscrew the locking nut. The best position to do this is hunched over the wheel while it sits on the ground. Whatever you do, DON'T open the cones at all while in this position. Just unlock it so you can unscrew everything with your fingers later.

Now this is the fast way to regrease your nsk bearings by just shooting grease in the bearings, screwing the cone back in, flipping the wheel over, hold the cone of the bearing you've just greased, grab the axle now facing up, unscrew it to reveal the other side bearings and pack them with grease. But the proper way is by taking everything off, clean all the cones, hub cups, bearings, repack with grease and reassemble. One great way to pack grease in wheel axles is by using a small feeder syringe. Only undo one side of the axle, this way you are sure that your axle is always centered when you reassemble it.

Reassembly is done screwing in the bearing cone back into place, but not too tight. Bearings should be able to roll smoothly without axle play. Tightened the lock nut into place and check again. If you ever hear crunching noises while turning the axle, dirt has gotten into your bearings and you should start all over again. To avoid this, make sure you work in a clean place. If everything is clean, you might have small pits in your bearing cones or cup. Your nachi bearings will be eaten away with time and it would be a good thing to replace those.

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