Engine bearings do many things. They support moving parts like crankshafts and connecting rods, but they also play a role in determining an engine's oil pressure and protect the crankshaft from being damaged by micron-sized particles. Improper bearing selection and/or sizing can result in problems ranging from poor oil consumption to all-out, catastrophic engine failure. Ensuring that the correct bearing size, and therefore proper clearances are selected, can be accomplished relatively easily and for surprisingly little money.
Plain, or sleeve, bearings are the most common type used today. Such bearings come in half-shell-shaped pairs that, when matched together at their parting faces, encompass a crankshaft's main journals and rod journals 360 degrees. Bearings can be made of a variety of different materials and always consist of a soft and a hard material cast, sintered or bonded to one another. Bearing material choice affects factors such as engine longevity, corrosion resistance and the ability to withstand fatigue and occasional detonation through increased load capacities. Babbitt, copper-lead alloy and aluminum are a few of the most widely used surface-coating materials of recent years. No matter which material is selected, all three are coated on top of a low-carbon, hardened steel backing-the material that makes up a good portion of the bearing shell, gives it its shape and receives the brunt of the load. Such coatings are applied in approximately 0.01-0.02-inch layers.
Babbitt is just about the oldest bearing surface coating and is probably the least used today. Its most useful trait is its high rate of embedability, a situation in which the bearings are more likely to pick up foreign particles, keeping them from scoring the crankshaft or floating in the oil film. The particles simply make their way around the bearing until they are completely covered by the material. Pull an old bearing out of an engine and you're sure to find speckles of particles thanks to the bearing's embedability characteristics. Of course, religious oil changes make a bearing's embedability less of an issue. Babbitt bearings also have a higher rate of conformity, which lends itself well to nitro and alcohol engines with extreme cylinder pressures. Copper-lead alloy bearings on the other hand are highly resistant to fatigue and extremely durable-not so much for the nitro engine but perfect for the typical race engine and most any street engine. Such bearings consist of a sponge-like layer of copper interspersed with lead particles. This combination allows for copper's strength and lead's embedability properties. Finally, aluminum bearings share many of the same characteristics with copper-lead alloy bearings but are even more resistant to corrosion. Aluminum bearings rank lower in deformability and embedability but that's becoming less of an issue with today's more sophisticated filtration systems. The aftermarket has also concocted a few different surface coatings including various alloys designed for even greater corrosion resistance and strength and there are certainly other less popular materials from years back such as tin-based and lead-based alloys as well as others that were cast directly as part of the connecting rods.
 ACL Race Bearings are comprised...  ACL Race Bearings are comprised of four metals: a hardened steel backing, a high-fatigue copper-lead lining, a nickel barrier, and a lead-tin-copper overlay for further fatigue resistance. What they don't have is the more conventional flash coating that gives other bearings their smooth appearance. The flash coating elimination improves heat transfer, which helps keep the bearing seated in its housing properly. |  Oil is distributed to the...  Oil is distributed to the crankshaft and connecting rods through holes like these in the block's main journals. It's important for a bearing to have enough provisions for adequate oil supply but not too much so as to compromise the bearing's strength. Sizing bearings appropriately will help ensure the correct oil pressure, and thus volume, is distributed throughout. |  Bearing crush refers to the...  Bearing crush refers to the distance the bearing's left and right parting faces protrude out of the journal. Bearing spread refers to the bearing's arc, which is slightly larger than the journal's. Both crush and spread keep the bearing firmly locked into place once everything is torqued properly. |