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What Is a Bearing Cap?

By Lori Kilchermann
Updated May 23, 2024
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A bearing cap can be described as a number of things, but the most common reference is the part of the engine block which contains the main bearings of a crankshaft. In an engine block, the part of the engine that the crankshaft runs through is split. Once the main caps are removed, the crankshaft can be set in the block on top of one-half of the main bearings. The other half of the main bearings are in the bearing cap and are placed in order onto the crankshaft and the corresponding other half of the main saddle. The bolts are then torqued to specification, thereby holding the crankshaft in place.

Piston rods also use a cap to hold the piston rod onto the crankshaft. Like the main bearings, the piston rod is split into two pieces: the rod and the cap. The rod bearings are divided into two pieces, with one half of the bearing going into the bottom of the rod and the other half going into the rod cap. The rod is slid into location onto the crankshaft rod through the journal. The rod bearing cap is placed onto the bottom of the rod and around the crankshaft, completing the union of the piston rod and the crankshaft with the connecting rod bolts being torqued to specification.

Perhaps tasked with the most important duty inside of an engine, the cap must be machined properly to allow the correct amount of oil to support the crankshaft and piston rod. The crankshaft does not actually ride on the bearing; in reality, the crankshaft rides on a layer of oil between the bearing and the crankshaft. This same principal is practiced between the rod bearing and the crankshaft. The proper machining and clearances between the bearing and the crankshaft is accomplished by the fit of the bearing cap. A cap that is too loose or too tight can cause an engine to fail prematurely.

While the piston rods are fitted and machined individually, the main bearing caps are machined as a unit in a process known as line honing. In line honing, each bearing cap is ground undersized and then torqued onto the engine block. The block is then placed into a machine, and a long hone is run through all of the caps, honing them to the proper specifications. This ensures that each cap is the perfect fit to the crankshaft.

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Discussion Comments
By Richie6018 — On Feb 15, 2014

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By anon343664 — On Aug 01, 2013

I spun the crankshaft bearings and when I got down to the problem, the bearing cap (U-shaped looking thing) was smaller than the other two. What causes this?

By NathanG — On Dec 07, 2011

@nony - That’s good advice. Of course, a lot of other things can go wrong with the car as well and a mechanic can’t just check everything as part of the routine maintenance. So you have to listen for symptoms.

I would assume that a bearing cap might be a problem if you start hearing strange engine noises. If the cap is loose or not properly fitted then it will not function well in allowing the right amount of oil into your engine block.

If you find yourself leaking oil, or having to fill oil more often than usual, I believe this might be an indication that something is wrong with the bearing cap as well. I am not a mechanic, but that would be my assumption based on the description read here.

By nony — On Dec 06, 2011

Sometimes it’s the smallest things that can cause the greatest amount of damage. I think this is especially true for automotive repairs.

The bearing grease cap is probably not something that you are inclined to think about. However, if it can be loose and destroy your engine, then it’s worth at least reading about it.

I didn’t realize until reading this article that it was that important and could go bad with normal wear and tear. I think you should make it a point when you get your car checked to have the mechanics go over the whole engine block, to the extent that they can, and ensure that the bearing caps are still okay.

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