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All You Ever Wanted To Know About Crank Bearings

Crank Bearings

Considering all that they do, crank bearings simply do not get the attention they deserve. Even the majority of performance enthusiasts do not fully understand the importance of crank bearings or what causes them to fail. To bring about a greater appreciation for this engine component, an explanation of what crank bearings do, how they are built, and what causes them to fail is needed.

What Crank Bearings Do

Crank bearings support the spinning crankshaft while simultaneously providing a near-frictionless surface that does not wear. Located in the crankcase, the crank bearings have two separate portions, an upper and lower part. The inner surface of the upper part generally has an oil groove, and the bearing generally has a hole in it that allows oil to pass through it to the crankshaft feed holes.

How Crank Bearings Are Built

Crank bearings fall into one of three types: Light, Intermediate, or Heavy Duty. They are designed as either a BiMetal bearing or a TriMetal bearing. BiMetals are composed of a steel backing attached to a single alloy layer. TriMetal bearings also have a steel backing, but they have a strong center layer with an outer layer of electroplated babbit.

Babbit is an aluminum-lead or copper-lead alloy. Originally used for the lining of light-duty bearings, a thin coating of babbit is combined with a strong copper-lead alloy to create a heavy-duty bearing. These bearings combine a low-friction surface with a strong core, and provide the best surface property with the highest load carrying capacity currently available.

What Causes Crank Bearings To Fail?

Normally, a thin protective film of engine oil separates the surfaces of the crankshaft and crank bearings. This film is created by the rotation of the crankshaft oil is pulled from passages on the high clearance side of the bearing to the low clearance side, creating a thin oil barrier between the upper and lower bearings and the shaft.

Under normal engine operation, the oil film is approximately .0001 to .0002 inches thick. This thickness can be cut by up to 50% by heavy loads and extreme temperatures. Because a strong protective oil film is always necessary to keep your crank bearings functioning properly, anything that disrupts the oil film will cause your crank bearings to fail prematurely.

While crank bearings are a wear component, is their deterioration is inevitable?

Their lifetime can be maximized by:

  • Regular lube
  • Oil
  • Filter

By using the correct lubricating oil and following the manufacturer's recommended maintenance schedule, bearing life will be maximized.

Hopefully, this description of crank bearings increases your understanding of crank bearings and gives you a greater appreciation for them. They truly are the unsung hero of your engine.