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If the main bearings are a little loose, all the oil in the valve chamber will leak out thru the mains when the car sets for a period of time. When you first start it, it takes a few seconds to pump oil back up into the valve chamber and gravity feed on down to the main bearings. This will result in a knocking sound for a few seconds.
This usually only happens with babbet bearings and not insert bearings.
This usually happens after the car sets overnight or longer and not after being shut off for just an hour or so.
10/40 or 20/50 oil may help.
I ran an engine for several years like this and it did not get any worse. I was too lazy to drop the pan and take up the bearings. Just run the engine slow and do not put any load on the engine until the oil gets pumped up.
Chris W.