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My two whole cents are that bearings are cheap, and if I'm in an engine to that degree, I'm just going to replace the bearings regardless of their condition. To eye ball bearings, you're looking for scratches, tears from hard particles, and color changes across the surface. Bearings are made of layers of soft material. As they wear through, the different colors become evident. Any color change is too much wear. The scratches are usually very obvious, and often the piece of grit that caused them will be lodged in the soft bearing. This helps to protect the harder metal in the crankshaft and such from damage. Also look for spun bearings. A rod bearing is normally jammed into the rod, and has the crankshaft rotating within it. When one gets sput, it will stick to the crankshaft, and spin around in the connecting rod. Very obvious by the scoring on the back of the bearing and on the rod. The rod is destroyed by this. Plastigage is a means for determining bearing clearance, when installed. You can't use this to measure the bearings or journals for selecting replacement sizes. To select replacements, you must use a micrometer and make actual measurements of the components. You use the plastigage after you've selected and assembled the first time to make sure that you selected the right size, and that they actually sold you the right size. Where things really bite a novice doing it alone here is in evaluating the condition of things like the rod. For if it's out of round, it can't support the bearing properly, and failure will most likely result. Same those marks on the crankshaft. Some are very significant. Presuming you're suffering from low oil pressure, other fairly common causes are:
Bad gauge,
Fouled line to gauge,
Worn oil pump,
Worn pressure relief valve,
Weak pressure relieve valve spring,
Stuck pressure relief valve,
Leaking pickup tube,
Excessive cam clearances,
Excessive wrist pin clearances,
Diluted oil,
Excessive engine heat
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