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Discussion Starter · #1 ·
I have seen basically two types of oil pressure situations on big block chevy's and am wondering why the difference.

1. High pressure 60 - 80 range........up to 80 range starting cold with 10/30 oil reducing to about 60 at idle warmed up and back up to the 70/80 range when reved up a bit.

2. Same as about except oil pressure drops to 15-20 pounds at idle warm and then goes up high when reved.

Anyone know what in the engine would be causing such a dramatic difference.

Is one situation better than the other or does it matter ?
 

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I have bled the oil pressure gage line before when getting erratic readings. My big block has the higher oil pressure range. never seen it do anything any different. Is the engine coolant temp doing the same thing or is it always steady once heated up?
Could the thermostat be sticking and the engine running hotter and causing the oil pressure to drop a little?

just some thoughts?
 

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There are basicly 3 factors that can give different results in different combinations.

1. Oil pump volume
2. Oil viscosity
3. Bearing clearances.

High volume pump, high oil viscosity, small bearing clearances, cold engine =
highest oil pressure.

Stock pump, low oil viscosity, large clearances, hot engine =
lowest pressure.
 

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Excessive internal leakage past the distributor housing?
 

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Discussion Starter · #5 ·
How can the oil leak past the distributor housing ?

Should the distributor always be fully seated down on the intake manifold ?

What about shimming between the distributor and intake to raise the distributor to get a better tooth contact pattern between the cam gear and distributor gear ?

How sensitive is distributor depth to the oil pressure/leakage question ?

Maybe we are on to something here !
 

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Distributor "depth" isn't the issue. The distributor housing has two machined surfaces towards the bottom end. Those fit--fairly loosely--into machined openings in the block casting. Part of the oil circuit is for oil to travel AROUND the section of the distributor housing and continue to the lifters. Any oil leaking past the lower machined surface provides lube for the distributor gear. Even so, there could be more leakage than is actually needed by the distributor/cam gear interface. Oil leaking past the upper surface does NOTHING useful. Both of these oil leaks are totally invisible--they are INTERNAL. The oil will just run down into the pan, and be pumped back up again.

This is why you cannot properly prime the oil system of a Chevy without having either a distributor housing in the hole, or a "special primer" that does effectively the same thing as the housing--seal that passage.

MSD distributors for Chevy have two O-ring seals on them. They must be used with a block that is de-burred properly so as to not cut the seals, and then you can positively prevent this oil leak. Note that on any distributor that has these O-rings, there is a tiny drilled passage in the distributor housing casting that squirts a metered amount of oil on the distributor gear. Install that distributor without lining up the oil squirter hole, and you might destroy the cam and distributor gear.
 
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