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PERA’s Core Corner: Back To the Basics On Engine Bearings

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Have you ever heard the phrase “old wives’ tales?” My father used it all the time whenever something didn’t seem to make sense to him for the moment.

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For instance, most everyone knows this one: hang a horseshoe over your door for good luck. Now most of the world will do so with the open part down, but in a few parts of Ireland and Britain they do the opposite, with the opening upward so as not to ever run out of good luck. Apparently the cosmic luck forces change based upon geography.

Like I said “old wives tales.” They can be a lot of fun but eventually you get to where the truth needs to be known. So, as the cartoon character Popeye would say just before his head would blow and his pipe would turn into a steam whistle, “that’s all I can stands and I can’t stands no more!”

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That is exactly the way I feel about how many people are confused about rod bearings without locator tabs or tangs.

But before we attempt to address that subject there are a couple of prerequisites that we have to get out of the way. Every once in a while you have to go back to the basics. I am going to give you an abbreviated “Cliffs Notes” explanation on two points, but I strongly recommend that you get this information in much greater detail from any of the major engine bearing manufacturers. Verify for yourself that what I am telling you is no “old wives’ tale.”

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Crush

The term “crush” refers to the outward force created by the portion of the bearing, which extends above the housing bore when the bearing halves are set into place. This “extra” material (Figure 1) holds the outside diameter of the bearings firmly against the housing bore when the assembly is torqued to specification. By increasing the surface contact between the bearing and connecting rod housing bore, crush minimizes bearing movement, helps to compensate for bore distortion and aids in heat transfer.

In simple terms: bearing crush is what holds the bearing in place. Think of it as putting 10 pounds of something into a 5 pound bag. The tang or locator tab on the shell that fits the saddle is only for locating the bearing during assembly.

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Figures 1 & 2

Figures 3

Eccentricity

“Eccentricity” refers to the variation in the inside diameter of a bearing assembly (Figure 2) when it is measured at different points around its bore. A properly designed engine bearing is not truly “round” when it is installed in the connecting rod or engine block.

Under operating loads, a rod or main housing bore will distort, pulling inward at the parting line between the upper and lower halves. To keep the bearing from contacting the crankshaft in these areas, most designs include additional clearance at each parting end of the bearing. As engine loads increase, so does the amount of distortion, thus race and heavy-duty bearings will require greater eccentricity than do passenger car bearings.

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The Chrysler 4.7L and 3.7L engines have transitioned to the “no tab” rod bearings but as best I have been able to ascertain the part numbers stayed the same. The “no tab” bearing is the service replacement as well as new production for these engines. They are the exact same bearing as can be seen in Figure 3. The aftermarket has also followed suit with the “no tab” bearing update, so everyone needs to get a grip and realize that the locator tab may be a thing of the past.

Here is an old wives tale I have been hearing within the builder segment: “The bearing tab keeps the rod bearing from spinning.” NOT! Tab-less connecting rod bearings are here to stay and will become the bearing style of choice for many OE’s in the upcoming future.

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If you’re wondering why these bearings and connecting rods have been changed, there’s a simple explanation: it’s less expensive to manufacture a bearing and connecting rod without a tab. You may actually find yourself in a position of having to install both types in the same engine because of inventory depletion of the early style tang type bearings. Fear not: it’s no big thing and the engine will never know the difference.

Special thanks to the engineering staffs of both Clevite and Federal-Mogul Corporation for their contributions to this column.

Also keep a watch for information of the third generation of EngineDataSource.com or EDS G3 and see how much more will soon be available.

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For technical questions, contact the Production Engine Remanufacturers Association (PERA) at: [email protected]

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