We Need You!

Caddyinfo Discussion Forum

Support Caddyinfo.com There are two easy ways to contribute to Caddyinfo.com
1) Caddyinfo Discussion Forum Contribution or Subcription click here
2) Direct contribution via Paypal, click the button below.

Join the forum
Join the Caddyinfo.com forum and share your knowledge of Cadillacs and Cadillac Culture.

Write Cadillac Articles Email or post your articles to share with other Cadillac enthusiasts!

Caddyinfo's Cadillac How-To Section

Cadillac How to 4.9L Bearing Thump

4.9L Bearing Thump

 4.9L Main Bearing Thump  SteveJ  8:29 am tuesday april 2, 2002

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.

 re: 4.9L Main Bearing Thump  bill  1:57 pm tuesday april 2, 2002
Steve,  Nice summary!!   One item to remember is that the main bearing clearances take into account any main bearing bore misalignment, any crank straightness or runout tolerances and any thermal distortion of the block or main bearing bores.  One would not want to put the tighter clearance service bearing in all of the main bearing locations as there would be insufficient clearance to allow the crank to rotate freely and there would be a high likelyhood of a main bearing "pinching" on the crank and "spinning" said bearing.  Not good.  The service (tighter fit) purple bearing shell should only be used on the front main and in no other location nor in any other location in conjunction with the front main bearing location.   The clearance in the other 4 main bearings is there for a reason.   Any misalignment of the system will have to be taken up more in the other 4 mains than in the front so the clearance must be there.   Actually, a tolerance stack of limit parts and limit operating conditions "proves" that the engine can't run with the tighter fit bearing shells in the number 1 postion but practice proves that it works ok and eliminates the bothersome noise.  If the reduced clearance would work on all engines and in all locations it would have been built that way from the factory....  Only use it in the front position

CaddyInfo Discussion Forum
Where Cadillac Fans meet to Discuss Cadillacs

Cadillac Acceleration Testing
0-60 mph, Quartermile, etc

Cadillac Performance Modifications
How do I add HP?

Ebay Auctions for Cadillac Wheels/Tires
Cardio-Doc knows Wheels

Archive of Cadillac Photos
Cars of People on the Board