Explaining Under- And Oversized Engine Bearings

When the subject of bearings and bearing clearance comes up, there is almost always a misunderstanding that takes place, thanks to the verbiage used in the discussion. So, today, with the help of King Engine Bearings, we’re not only going to clear the air on the terminology of under- and oversize bearings, but also some of the ways you can easily fine-tune your clearances to the half-thousandth of an inch.

Oversized Versus Undersized

The first, and most important item to clear up before we even get started is the terminology we’re going to be using. All engine bearings come in the standard (or STD) sizing for the specific application. That bearing size is designed to fall 100 percent within factory specifications for bearing housing diameter (be they main bearings or rod bearings), crankshaft journal diameter (be it a main journal or rod journal), and factory-specified oil clearance. So, if absolutely everything in your engine is to factory specifications and tolerances, standard bearings are perfect for you.

However, we live in a less-than-perfect world, and a lot of times we’re doing things that would make factory engineers cringe (officially, anyway). To address the variances in sizing and to allow for component wear, alternate-sized bearings exist. These are referred to as undersized or oversized bearings. Now, where the confusion comes in here, is that in both bearing designs, there is more material than standard. Because A) bearings are designed to work at a minimum material thickness, and making them thinner would make them weaker, and B) if your journal is too large, you can simply machine the journal.

That said, oversized bearings do exist in the King lineup. “In an oversized bearing, material is added to the outside, increasing the outside diameter,” explains King Bearings’ Guy Haynie. “Those bearings are used when material has been removed from the engine block or rods.” These bearings, in a standard bore diameter with a standard-sized journal, will reduce the oil clearance by .001 inch (.0005 inch per bearing shell). Conversely, if your main bore or rod’s big end diameter is slightly oversized, they will bring oil clearance to factory specs.

The other option is undersized bearings. “In an undersize bearing, material is added to the inside, decreasing the inside diameter,” explains Haynie. “They are used when material has been removed from…