Rod bearings are thin curved shells of tri-metal or bi-metal material that ride between the connecting rod big end and the crankshaft journal. They look passive, but their installation geometry is doing constant battle with combustion forces trying to spin them in their bore. Get the geometry wrong and you spin a bearing — a catastrophic failure that usually destroys the crank, the rod, and your weekend.

Crush is the secret. When you torque the rod cap down, each half-shell is intentionally manufactured slightly taller than half the bore circumference — by roughly 0.001"–0.003" per shell. As the cap pulls down, the two halves contact each other before the cap fully seats, and the final torque squeezes the shells radially outward into the rod bore. This radial preload (crush) is what locks the bearing against the housing through friction. Without crush, the bearing spins. With too much crush, the bearing distorts inward and the oil clearance disappears.

Spread is the other half of the trick. Each free shell, sitting on a bench, is sprung open so its outer diameter is slightly larger than the rod bore — typically 0.005"–0.020" of spread. When you snap the shell into the rod, it grips the housing on its own before the cap ever goes on. Spread holds the shell in place during assembly; crush holds it in place during operation.

The locating tang is the small bent tab on one edge of each shell. Critically, the tang does not carry load — it only indexes the shell during assembly so the oil hole lines up and the bearing isn't installed backward. Both tangs must sit on the same side of the rod, in the milled tang groove. A common rookie mistake: installing the upper and lower shells with tangs on opposite sides, which puts the parting lines under load and starves the bearing of oil.

Real-world example: The Honda K20 rod bearing spec calls for ~0.0008"–0.0015" oil clearance using color-coded bearings (yellow, green, brown, pink, etc.). Honda matches journal size to rod bore size with a color chart — pick the wrong color and you'll either seize the bearing or hammer it to death from excess clearance.

Plastigage rule of thumb: Lay a strip of Plastigage across the journal, torque the cap to spec, then remove and measure the smashed width against the wrapper scale. Target clearance is roughly 0.001" per inch of journal diameter — so a 2.0" journal wants about 0.002" clearance. Tighter for race builds, looser for high-mileage street engines.