Main Bearing Caps
Crankshaft bearings are coated with soft metal. The engine's oil pump sends pressurized oil through holes drilled through the crankshaft to the bearing surface. They're designed to allow a thin film of oil to flow through the clearance between the bearing and the crankshaft journal. Fresh pressurized oil flows into the gap and past the bearings edges. This oil creates a cushion that absorbs the downward force created by the engine's power stroke.
Excessive wear results in a knocking or rapping sound. It has worn past specifications, and the gap has increased. This wear left the bearing, and the journal unprotected.
Remove the oil pan to inspect the bearings. If the coating is worn and brass is present on the bearing surface or in the oil pan, the bearings need replacement. The main caps and journals are inspected and measured before replacing the worn bearings. The main caps become oval from the intense downward pressures imposed on them by the engine's crankshaft.
Inspection: A line boring tool bores the main bore through the bottom of the engine block and the main bearing caps. Each cap is bolted in place during the process. It isn't perfect, especially toward the end of the boring process. This is why it's so important to keep these caps in order. They're numbered with arrows pointing toward the front of the engine. Whenever working with an engine part, arrows or indented dots typically point toward the front of the engine. Check the appropriate manual for proper procedures before taking measurements.
Crankshaft end-play is measured with a dial indicator mounted on the engine and the tip of the indicator on the crankshaft flange. Use a pry bar or a large screwdriver to move the crankshaft back and forth. It's important to check these measurements before and after a rebuild. Clearance should be minimal.