Hi guys,
Wally is right on the money about oil pressure not being related to bearing clearence. If the bearings had worn enough to cause an oil pressure problem, they would be into the copper and causing a whole lot of more serious problems. More than likely if the oil pressure is going down to near 0 at idle, something else is wrong. A stuck relief valve in the pump will do it, also on a Small Block like rancon's one of the front oil galley plugs could have come out and fallen into the pan. This will cause the same problem. The size of the bearings (10 - 20 -30 )has absolutely no effect on the oil pressure unless you have the wrong size bearings in the motor. I had a friend build a 302 Ford once and put .010 bearings on a crank that was ground .020 and beleive it or not, it had great oil pressure when cold but dropped to about 10 pounds hot, so this is another possibility.
By the way rancon, was your crankshaft reground ? If so ask your engine builder what the clearences were when he assembled the motor. It would be interesting to hear the answer. I just finished a 350 motor for a 85 MonteCarlo SS and it has 70lbs cold and 40 hot at idle, this is with a stock pump. My 85 Blazer still had 30 lbs hot idle oil pressure after 135,000 miles. Our shop S-10 with a 2.8 has 170K miles on it and has never been touched, it runs 30 lbs hot at idle with 5w30 Mobile 1. I would not accept anything less for your motor. You should not have to run heavier oil to get correct oil pressure. 10w30 should give you at least 30lbs hot idle oil pressure with a stock pump, even more with a high voloume or high pressure pump. I would insist on it being fixed now rather than have a problem later !
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Bill Koustenis
Advanced Automotive Machine
Waldorf Md