From http://craig.backfire.ca/pages/autos/boxchevy/350/crank-reinstall
Grit in the Bearings
A major mistake was made when I installed the crankshaft, rods, and pistons the first time. I forgot to clean out the oil passages in the crankshaft and in the engine block. The engine sat over the winter and all of the grit that was in the passages found its way into the main bearings. As was the case before the rebuild even started, the bearings needed to be replaced. Luckily, the crankshaft itself and all of the rod bearings were unharmed.
All of the main bearings were damaged from grit that was in the oil passages.
I cleaned out the passages with a rifle brush, which I found would only remove larger bits. The finer grit came out when I used large foam-tipped things that look a lot like Q-Tips. I soaked them in brake cleaner to help break down the grit, and I continued to make passes until the brushes came out clean.
Cleaning the passages started with a rifle brush and ended with foam brushes.
Before reinstalling the crankshaft, all of the oil passages in the block were cleaned using the methods above.
Rear Main Seal Issues
Another issue that I knew about but chose to ignore was the fact that the crankshaft still had some scoring where the rear main seal touches it. I took the opportunity to replace the rear main seal with an offset seal which would be in contact with smooth areas of the crankshaft. The seal is made by Fel-Pro, part number BS11829.
The crankshaft was not smooth where the rear main seal was in contact with it.
The new seal is offset to allow contact with sections of the crank which are not worn.
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