Every crankshaft has main bearings that handle radial loads — the up-and-down pounding from combustion. But there's another bearing most people never think about until it fails catastrophically: the thrust bearing. It controls axial movement, keeping the crankshaft from sliding forward and backward inside the block.

The thrust bearing is usually a flanged version of one of the main bearings (typically the center or rearmost main on most engines, though Ford small blocks famously use the #3 main). Instead of just a curved shell, it has flat thrust faces on the sides that ride against machined surfaces on the crankshaft cheeks. Total endplay is typically 0.004"–0.010" — measured with a dial indicator on the snout while prying the crank fore and aft.

Where the axial load comes from:

• Clutch engagement — every time you push the pedal, the release bearing pushes the pressure plate fingers, which pushes the crankshaft forward through the pilot bearing. On a manual car, this happens thousands of times per drive.
• Torque converter expansion — automatic transmission converters balloon under hydraulic pressure and push the crank forward. This is the #1 thrust bearing killer.
• Helical gear loads from timing gears or accessory drives create small axial components.

The Ford 4.0L SOHC and GM 4T65E story: The infamous "thrust bearing failure" on late-'90s/early-'00s GM transverse V6s with the 4T65E transmission killed engines by the thousands. The torque converter's apply pressure was poorly regulated, creating constant forward thrust on the crank. The thrust faces wore through, the crank walked forward, and eventually the timing chain skipped or the front seal blew out spraying oil everywhere. Symptom: a faint clunk when shifting from Park to Drive, then catastrophic failure.

Diagnosing it yourself: With the engine running and transmission in neutral, have someone press the brake hard while you watch the crank pulley with a flashlight. Movement greater than about 0.015" front-to-back means the thrust bearing is toast. Rebuild now or replace the engine later.

Rule of thumb for endplay: Take your bore diameter in inches, multiply by 0.002, and you're in the right ballpark for maximum acceptable endplay. A 4" bore engine should run roughly 0.008" max — beyond that, oil film breaks down on the thrust faces and metal-to-metal contact begins.

Performance engines with high-stall converters or heavy clutches often upgrade to thrust washers — separate hardened steel pieces that handle axial loads independently of the radial main bearing, dramatically increasing load capacity.