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.

Here you can see the various bearing sizes offered by King in their lineup.

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 the crankshaft journals.” Undersized bearings come in far more size options. Because not only do they come in the same .001-inch variation as the oversized bearings, but also in .010-inch, .020-inch, and .030-inch undersize variations, for journals that have been turned down .010-, .020- or .030-inch, respectively.

To further complicate the issue, within each of those undersize variants an oversize variation (denoted by an X in the size) exists, as well as a variant with .001 inch less clearance. What that means is that, in the King bearing lineup, options exist to go from standard, or to gain an additional .001 inch of (or “loosen up”) oil clearance. On the other side of the equation, the available undersize options are .001, .009, .010, .011, .019, .020, .021, and .030-inch of less clearance, giving you the ability to “tighten up” your clearances.

These .001-inch-undersize LS rod bearings allowed us to mix with standard-size bearing halves to pull out .0005 inch of oil clearance from our rods.

Fine Tuning Those Clearances

As you might have seen recently in LS5.0’s short-block build, we found ourselves in the middle of the clearance range in our rods. We wanted to tighten up our clearances by about .0005 (half a thousandth) of an inch. Looking at the chart, our only option appeared to be reducing the clearance by .001 inch, or twice what we wanted to take out. So, what to do?

That is where mixing bearing shells comes in. By taking a standard rod bearing lower half, and a .001-inch undersized upper half rod bearing shell, and combining them in the same connecting rod bore, our clearance is increased by half a thousandth of an inch. That brings our tolerances exactly where we want them. This is common practice among engine builders who are constantly chasing ten-thousandths of an inch, and don’t typically settle for “good enough.”

This kind of granularity gives you the ability to fine-tune your oil clearances without a trip to the machine shop the alter journal or housing diameters, with the only cost to you being a second set of bearings. A few best practices to follow, are to place the undersized bearing halves in the same positions (either all upper or all lower, not mixing and matching) throughout the engines. The next is that this is a method to get .0005 inch of variation. It’s not recommended to use bigger variances, like trying to get .005 inch of clearance change by using a standard and .010-under bearing half.

By understanding how bearing sizing works, you are not only able to understand your options when assembling your next engine, and potentially avoid another trip to the machine shop if your clearances aren’t exactly where you want them, but this knowledge can also allow you to fine-tune your bearing clearances like a pro.