Low Oil Pressure
A worn oil pump will cause low oil pressure. They wear in time, and the clearance between the pump housing and the gear gets excessive. Pumps also fail if debris in the oil gets caught in the housing, causing premature wear and damage. A clogged pickup screen results in a restriction to oil flow and a loss of oil pressure.
Bearings are designed to wear, and are replaced when rebuilding an engine. The growing gap between the bearing and the journal increases oil flow. This increasing gap lowers oil pressure. These two are proportionate to each other, the larger the gap between the bearing and the journal, the lower the oil pressure.
Worn crankshaft, camshaft, and rod bearings cause a lower oil pressure. There are different specifications and tolerances according to the application. This is usually no more than .001 inch off the diameter of the bearing.
All of these bearings should also be checked for scoring and tapered wear. This may indicate a problem with a worn or damaged part such as a twisted rod.