Every rotating and reciprocating component in your engine rides on a thin film of oil suspended between precision-fitted bearing shells. Get the bearing wrong and you'll spin a bearing, starve a journal, or grenade a bottom end. Here's how they actually work.

Types of engine bearings:

• Main bearings — support the crankshaft in the block. Typically split shells (upper and lower halves) that seat into machined saddles.
• Rod bearings — sit in the big end of connecting rods, carrying the reciprocating load from pistons to crank.
• Cam bearings — typically one-piece shells pressed into the block or head, supporting the camshaft journals.
• Thrust bearings — control axial (fore-aft) movement of the crankshaft. Usually flanged main bearing shells or separate thrust washers.

Bearing materials are layered by design. A modern tri-metal bearing has a steel backing for structural rigidity, a copper-lead or aluminum intermediate layer for load capacity, and a soft overlay (often lead-tin-copper or polymer) that provides embeddability — the ability to absorb small debris particles without scoring the crankshaft journal. Bimetal bearings skip the overlay and use aluminum-tin alloys directly, trading some embeddability for durability. Race engines often run trimetal with tighter clearances and higher-viscosity oil.

Oil clearance is everything. The gap between the bearing surface and the journal determines oil film thickness, oil flow rate, and heat dissipation. Too tight and the film breaks down under load, causing metal-to-metal contact. Too loose and oil pressure drops, the bearing hammers, and you get knock at the bottom end.

Rule of thumb: target oil clearance of roughly 0.001 inch per inch of journal diameter. A crankshaft main journal at 2.500" diameter gets about 0.0025" clearance. Rod bearings typically run slightly tighter — around 0.0002" less — because they see higher unit loading. Measure with Plastigage strips or, better, a bore gauge and micrometer for sub-tenth accuracy.

Real-world example: The GM LS engine uses bi-metal aluminum rod bearings from the factory with a specified clearance of 0.0009"–0.0025". Builders pushing 700+ HP on boosted LS combinations commonly switch to ACL or King tri-metal bearings, target 0.0022"–0.0028" clearance, and run 10W-40 oil to maintain film strength under high cylinder pressure loads. Going below 0.0018" on a boosted LS rod bearing is asking for trouble — thermal expansion under load closes that gap fast.

Crush and locating tangs matter too. Bearing shells are manufactured slightly larger than the bore so that when the cap is torqued down, the shell is compressed (crushed) into firm contact with the housing. This prevents the shell from spinning. The small tang on the bearing's parting line is only a locating aid during assembly — it does not hold the bearing in place.