This is some general information on bearings. It is not specific to the EX250, or even to motorcycles. The following information applies to bearings that have a rolling element (balls or rollers), not sleeve bearings.
- Outer ring: the outer part of the bearing
- Inner ring: the inner part of the bearing
- Rolling element: the balls or rollers that are between the inner and outer ring
- Cage: the part that separates the balls or rollers. It can be made of brass, steel, plastic, or other materials. They all work.
- Seals and shields: rubber and/or metal that is attached to one ring, usually the outer, designed to keep trash out and lubricant in
- Internal clearance: the space between the rings and the rolling element
- Ball bearing: the rolling element consists of round balls
- Roller bearing: the rolling element consists of rollers that have various shapes. The most common shape you see in motorcycles, trucks, and trailers are tapered roller bearings, often called Timken bearings because Timken invented and makes most of them. You may also see needle bearings, which have long, thin rollers, used as swingarm bearings. They are also used in some transmissions.
- Cup: the outer ring of a tapered roller (Timken) bearing
- Cone: the inner ring with rollers and cage of a Timken bearing
- Most ball bearings have metric dimensions; it doesn't matter where the machine they are in was made. The last two numbers of the part number times 5 gives the bore (id) of the bearing on sizes that have a 20mm bore and larger. Example: A 6305 bearing has a 25mm bore. 5x5=25. A 6209 has a 45mm bore. 9x5=45. (This does not apply to tapered roller bearings.)
- "Internal clearance”, or fit: Many people get confused when they find out that a bearing can be ordered with different internal clearances (which is the space between the races and the rolling element) and think that one is better than another. A ball bearing is usually supplied with either "normal" or "c3" internal clearance. A ball bearing with "c3" fit simply has more internal clearance (space) than a bearing with "normal" clearance.
- You can use either when replacing a bearing. The main thing to remember is that THE QUALITY IS THE SAME. If given a choice, use the c3 fit; it has more internal clearance to allow for expansion from heat and improper shaft and housing dimensions.
- Ball bearings can be open or have a seal or shield on one or both sides. The dimensions are the same as an open bearing. If a bearing has a seal/shield on both sides, it does not have to be re-lubricated. The lube that comes from the factory is designed to last for the life of the bearing. This does not mean that a high pressure washer will not blast water in and grease out, so avoid a direct blast from a car wash if you use one.
- A replacement bearing is often less expensive from a bearing distributor than from a motorcycle shop. The best way to identify a bearing is with the part number that is stamped on the bearing, not an OEM item number. Call a distributor, give him the part number, and add any information that may help, such as “a 6305 bearing with a seal on one side”.
- Note on Timken bearings: A Timken cone will fit several different cups, and a cup will fit several different cones, so you have to need the cup and cone part numbers.
- Rule of thumb: The rotating bearing ring will normally be an interference (or press) fit, and the stationary ring will be a slip (not sloppy) fit. For example, the outer ring on a wheel bearing (the wheel rotates) is a tight or press fit, and the axle slips through the inner ring. The inner ring on an electric motor shaft (rotating) is a press fit, and the outer ring is a slip fit into the end bell of the motor.
- Heat to expand a part or cold to contract a part may be needed, but many bearings can be pressed into place. Use a steel item such as a socket to press the ring that has a tight fit; it should only make contact with the ring that has the tight fit. Never push one ring to press the other ring into place; this requires the rolling element to transfer the pressure from one ring to the other and can dent the roller path, leading to premature failure.
- Tapered roller bearings are normally used in pairs and have their internal clearance set by how tight they are pushed toward each other when a nut is tightened. For example, on a trailer wheel, tighten the nut while turning the wheel until you feel resistance, then back off one notch and insert the cotter pin. Remember, there has to be some clearance.
- Never use a brass hammer or brass bushing to install a bearing. It can chip and leave a small brass particle in the bearing.
- The major bearing manufacturers all have factories in several countries, and the quality from each is excellent. These include SKF, Timken, Torrington, Fafnir, NSK, NTN, RBC, McGill, and a few others. Most of these companies use bearing grade steel that is made by Timken, even though they are competitors of Timken.
- Avoid bearings from China, Korea (KBC) and third world countries.