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Almost all quarter midget bearings are shielded bearings. This means that they have a protective shield to keep dirt out and therefore are relatively low maintenance. However, in order to be low maintenance or maintenance free they have to use a heavy grease in the bearings. This heavy grease is great for lubrication; unfortunately, it creates unwanted drag. I have asked a lot of people about how to clean and maintain my bearings and the fastest racers all seem to prescribe to this method (or one similiar). Be warned that if you choose this method you will have to work a little harder to maintain your bearings, but it will be worth it! 1) Start by removing the bearings from the birdcages or fron hubs (this is only necessary if this is the first time the bearings have been cleaned). If the birdcages and hubs are aluminum you can easily remove the bearings by applying a little heat to the birdcage/hub with a torch. The aluminum will expand and the bearings will drop right out. 2) Remove the dust shields from one side of each bearing. Most axle bearings are going to have metal shields on both sides. They were not designed to be removed, and the can not be put back once they have been taken out. It is important that you only remove one side (I'll explain later). The front wheel hubs on the other hand probably have a rubber shield on at least one side. This shield can be replaced once the bearing has been cleaned and re-lubed. Using a small pin or razor blade carefully pry the shield up by placing the pin/blade between the inner race and the shield. Be careful not to make contact with the balls of the bearing that are underneath the shield. Work your away around the edge of the shield prying up a little at a time until it pops out. 3) Clean the bearings. I have found it best to use some sort of solvent such as brake cleaner. You may use other cleaners/degreasers, but be careful of any that contain water. Taking the necessary safety precautions (gloves, goggles, etc) clean all of the grease out of the bearing. I have found it best to let the bearings soak for a while in a metal or glass container (careful not to use plastic because the solvent may eat through it) and let the solvent do the work. Once you have cleaned every last bit of grease and dirt out of the bearings dry them with compressed air. Another warning: Don't get carried away here and let your bearings spin up to a high RPM while you are blowing the air into them. Remember, they don't have any lubrication at this point and you could damage them. 4) Re-lube the bearings. For this you are going to want to use a very lighweight oil. My personal preference is Tri-Flow. It is available at most bicycle shops and numerous places online. It is sort of expensive, but you don't use much of it, so it should last quite a while. If the bearings had rubber shileds you can pop them back in now. 5) Reinstall the bearings. I use a couple of pieces of wood in the bench vise to handle this job. The important thing to remember here is to have the open side of the bearing (the side without the shield) facing inward. This will prevent dirt from getting into the bearing because the bearing will be protected by the birdcage or hub. After the initial cleaning it is not necessary to remove the bearings for cleaning. I know several people that leave them in the birdcages for cleaning once the shields have been removed. Make sure to keep the bearings lubed up and they will role like butter.
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