Back to Basics: Engine Bearings - Engine Builder Magazine

Back to Basics: Engine Bearings

Most everyone knows this wives tale: 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!"

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."

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.

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.

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.

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.The

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