Question: I am trying to get my oil pressure up a bit in my 401 V-8. I have some information about gasket thickness being part of the contributing factor for low hot-oil pressure. What is your take?
I remember the first time I fired the engine I broke the distributor gear because I didn't use a gasket at all-just gasket sealer, which bound up the pump. So I have to take it apart again and I will try and see if I can get some different thicknesses of gaskets, if that's what I should try. I moved to 20/50 in the engine and it didn't boost it too much. It looks like the pump is nice and tight to the sides of the oil-pump chamber, and it doesn't look like much scoring inside either. This is a high-performance engine with very low mileage.
Answer: What? You broke the drive? That means the engine either ran or cranked over without any oil pressure! Shame on you-you're supposed to first spin the pump as you assemble it to make sure it is not binding! After that, I like to pack the pump in Vaseline, not grease, as this eliminates any air and makes for a better prime. You did pressure-lube all the oil passages before you started the motor, right?
AMC and Buick V-motors all have oil delivery problems due to the external pump, long passageways, and too-small oil channels. On the Buicks, we would drill and blend the oil holes from pump to block, and make up a spacer plate that, of all things, used gears from a 409 Chevy that were about a third longer. Oiling was one of the biggest problems that Team Penske had when they ran AMCs in TransAm races. They did a lot of development work to make the engines live.
Pump clearance is an important aspect of oil pressure as well as engine clearances. Start with the pump first. It's been a long time since I worked on an AMC engine, but as I recall, there should be about 0.001 to 0.002 inch of clearance on the sides-maybe even as low as 0.0005 if you can get it, but that is hard to measure. Use a narrow feeler gauge between the housing and the gears, not a wide one, to ensure accuracy. You should rotate the pump in the housing and check each gear for proper clearance-maybe even at different spots in the housing. I have seen new gears not cut equally on all the teeth. Examine the housing real closely. It doesn't take much wear or scoring in the aluminum housing to gain clearance and lose oil pressure. You may end up buying a new pump, gears, and/or a housing/timing cover. Believe it or not, I have even seen a difference in clearance with different brands of gears. Remember, we are talking about tight clearances here.
Measuring the cover clearance is a bit more tricky. I like to use a piece of Plastigage across the top of each gear, then put the cover on and tighten it down, and then measure the flattened Plastigage with the scale provided. You can use a feeler gauge and a straight edge for this, but it's hard to get an accurate measurement. The clearance should be about 0.002 inch for the best oil pressure. Excessive clearance can be reduced by filing the pump housing to remove some material, but if you have to do this, do it very carefully in order to maintain a flat surface. Better that it be put in a mill and machined off, but that's a real pain to make a fixture to hold it in the proper position.
If you don't have enough clearance, then it can be gained with different thickness of gaskets. You want to use a fairly hard gasket material to limit compression and a change in clearance.
Be very careful using any type of sealer on an oil pump. If you do use a sealer, run a flat mill file over the pump-housing sealing ends to make sure it is square and true. Careful here-it has to be flat and true. The small lines hold the sealer in. Then use some type of gasket sealer, like "Copper Coat" in a spray can. If you're real careful, you can use a thick layer of Aviation Permatex, but you sure don't want to get any excess into the pump.
Also remember that there is a relief valve. Make sure it is not sticking in the open position and allowing oil to flow back into the pump. It most likely has a spring that is set for somewhere around 65 to 75 pounds. This doesn't mean that the pressure will be that high-it is just that it will open at that high, like on a cold-morning start with heavy oil.
If for some reason you think that you need more maximum oil pressure, don't stretch the spring. Use a shim, or maybe like washers, between the screw on the cap and the spring. But remember, it's the pump that makes the pressure.
In reality, all you need is about 10 psi for every 1,000 rpm or so. However, you don't want to load an engine hard at, say, 2,000 rpm and only 20 psi, as this will actually cause oil shear. Part of the oil pressure that the bearing sees is from the hydraulic wedge effect of the crank turning and trying to get closer to the bearing from the imposed load.
If the pump checks out properly, then the low oil pressure could be from too much rod or main bearing clearance, including side clearance on the rods. Excessive side clearance will let too much oil bleed out. This can depend on just how careful in taking measurements your engine builder was.
It could be that you "added clearance" to the engine when you cranked it without oil pressure. It doesn't take much, especially with a performance engine. My father-in-law, who was an engine builder of note, was a real stickler on always pressure-lubing an engine before starting, making sure that he would have oil pressure showing on a gauge, and the engine should fire, like, right now! He saw many of his engines damaged by lack of oil pressure when first started.