Like clearance, alignment is also extremely important especially for high performance applications. Even if not ground to provide additional clearance, camshaft bearing journals should be polished to a surface finish of 10 micro-inches Ra or better with the camshaft rotating in the same direction it will rotate in the engine. Bearing ID's may be reamed, but the most practical means is to adjust camshaft journal diameters by grinding.
Honing the ID's of cam bearings to increase clearance is not recommended because hone grit may become embedded in bearing surfaces which will cause shaft wear. Clearances must be measured at installation. Because of the stack up of tolerances on the block, shaft and bearing it is impossible to control clearance to this range in the manufacture of the bearing alone. Optimum clearance range for high performance applications is. These stronger bearings will not wear in rapidly to make their own clearance like softer materials. Sufficient clearance is necessary in the initial installation. Following are some recommendations to help optimize performance when using aluminum alloy camshaft bearings. Conditions such as cleanliness, alignment, clearances, journal surface finishes and lubrication must all be controlled more closely. Typically, whenever a higher level of loading is encountered, greater precision is required to maintain reliability.
This is typical of the compromises or trade offs that are frequently necessary when selecting a bearing material to suit the requirements of a specific application and in this case, higher loading. The aluminum alloy is harder, making it somewhat less compatible with dirt, misalignment and marginal lubrication. However, this added strength is obtained at the expense of some of the more forgiving properties of babbitt. This aluminum alloy is much stronger than babbitt and will withstand several times the load which causes babbitt to fatigue or extrude. To meet the demands of higher loads and operating temperatures in modern engines as well as the requirements imposed by high performance, babbitt has been replaced by an alloy of aluminum. Fatigue can be identified by craters in the bearing surface where sections of lining material have flaked out. When babbitt cam bearings are installed under these demanding conditions, the lining may extrude or fatigue. Babbitt is limited in its ability to survive under these conditions due to its relatively low strength. The trend in modern engines has been toward higher operating temperatures and higher valve train loads. As a bearing surface layer, babbitt possesses the desirable properties necessary to survive under adverse conditions such as foreign particle contamination, misalignment and marginal lubrication on start up.
Babbitt is a soft slippery material made up primarily of lead and tin and is quite similar to solder. For many years, nearly all camshaft bearings were manufactured with a lining of babbitt.
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