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 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. The technician in the question is hearing the front most bearing making this knock. It has worn past specifications and the gap has increased. This left the bearing and the journal unprotected.
The oil pan must be removed to inspect the bearings. If the coating is worn and brass is present on the bearing surface or in the oil pan, the bearings must be replaced. 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 to the front of the engine. Check the appropriate manual for proper procedures before any measurements are taken.
There are different tools for measuring the bore. A bore gauge is a great tool for this purpose. Many use a telescopic gauge along with an outside micrometer or caliper for similar results. Crankshaft end play is measured with a dial indicator mounted on the engine and the indicators tip 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, a few thousandths at the most.