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The 4.9 Cadillac engine has a peculiarity in that the front main bearing can make a distressing noise which can cause knowledgeable owners to think it needs a bearing job. Those familiar with the sound of a bad bearing will be certain they hear a main bearing knock which requires fixing before it becomes worse.

The good news here is that the bearing need not be fixed and that it will lead a long and happy life making noise all the way. If your engine fails it will not be the front main bearing that causes it. Those in the know refer to the noise as a "thump" to distinguish it from a knock. A knock is bad but a thump is benign.

The engine uses a serpentine belt to drive the accessories and has an automatic tensioner to keep belt pressure correct. This particular engine design has the serpentine belt lifting almost straight up on the front damper which would be no problem except that whenever cylinder 1 or 2 fires they push downward. The lifting of the belt and the firing of the cylinders act against each. This causes the slightest oscillation of the crankshaft in the bearing due to the accumulation of normal clearances. The belt lifts the damper and crankshaft upward followed by the firing cylinder pushing it downward and a thump occurs.

People studying the factory service manual for late night reading will notice that the specified clearance for replacement of the No.1 bearing is "greater than .002 inch" while for all other main bearings it is "greater than .004 inch." You may draw your own conclusion as to why this is true. Just for comparison the service clearance is normally .0008 to .0015 inch on the main bearings.

Extensive testing and reports from the field show that the No 1 main bearing thump has no ill effect on durability or longevity. It will simply make noise. The noise may vary with the temperature of the engine, viscosity of the oil, and speed of the engine. Some people have reported that the thinner nature of synthetic oils tends to make the noise louder in which case using the factory recommended 10w30 mineral oil would make good sense.

Should your engine thump predictably at idle it is easy to confirm the diagnosis. Take a breaker bar and as the engine idles thumping away unload the belt tensioner allowing the belt to stop turning or at least slip easily. The thump should magically disappear. Engines that thump only when under a slight load can also be tested but to do so is dangerous to careless persons so safety is an important concern. A second person will be needed to sit in the driver's seat and step on the brake while also slightly speeding up the engine until the thump is heard. The belt can then be unloaded as above. Care should be taken not to stand in front of the car or have a foot in front of the tire. Anytime a person reaches inside the engine compartment of a running engine care should be taken to avoid moving parts.

There are of course people who cannot abide by this noise no matter how harmless it may be. For them the Cadillac dealer has the correct oversize replacement bearing in stock. Reportedly the replacement bearing is "purple" as GM designates its bearing sizes by color.