Thread: Oil pressure and main bearings.

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I've got a high milage 350 in my truck and since I bought it, about 10,000 miles ago, it has had about 5 psi oil pressure at idle; that is if the guage is linear and accurate. The pressure tops out at about 35 psi when the engine is cold, but that is what the pressure is regulated for; I get 30 psi when I'm driving. I am starting to hear and feel the crank walking in the bearings at idle and what I want to know is, can I fix this by replacing the oil pump with a high volume unit, or do I have to replace the mains. As the oil pressure picks up with rpm, the motor runs smoother and smoother, all of the way to the redline. I know I should just rebuild the motor, but I am on a college student budget of 0 dollars right now, that and it runs like a champ, no smoke or anything. Just a little positive crank case pressure when going up long steep hills at 65 mph - thats with 3.73 gears and no overdrive too.

I can buy a set of Clevite bearings from jegs for around 50 bucks, I guess I could make some sacrifices to be able to afford that. They come in undersize increments of .010" Seems like the GM bearings would be the way to go. What do they cost for the rods and crank? I don't have to turn the crank, do I? Seems like the most expensive part of this job is going to be getting a good micrometer. And, what should the clearances be? Trouble is, I've got to make a trip home to get all of my tools, and that is about 180 miles round trip. I was kind of hoping to be able to replace the oil pump because that would take fewer tools and I wouldn't need to buy an engine stand and micrometer.

For the trip home you might put some thicker oil in it if it's possible. Example if you are running 10-30 switch to 10-40.

Abe

There's not much sense in ordering parts until you take it apart and find out what kind of shape the engine is in... Chances are if you hear and feel the crank banging around, it's going to need to be turned, you won't know until you have it out. The major bearings that wear out also include your cam bearings, and if the motors out, a set of those should also be included. Additionally, having a machine shop check the rod big ends for out of round (most machine shops will not charge for this or charge very little just to check them) and resizing as necessary is a good idea. On any high milage overhaul/rebuild a new oil pump is also a good move.

As far as your bearing clearances go, although I could tell what they should be, I think the best advice I ever got was from the head mechanic in the shop I started out at 35 years ago when I asked the same question "Always get the shop manual and look clearances and torque specs up for yourself, it's too expensive to make a mistake on"

This was also the the first person I ever heard say "how come people never have the time or money to do it right, but always can do it over?"

Good luck.

I'm already running 20-50, so I can't really go up on the viscosity. The crank isn't banging in the bearings yet, it just looks like the engine is idling rough. There is no metal in the oil yet, so I don't think the crank is damaged. I also feel confident the engine will last another few thousand miles, it's only rough at idle because of the low oil pressure. My plan was to pull the crank, measure the journals, go to GM, and buy whatever undersize it would take to put me within spec. Then, I'll start saving my money for a rebuild. Chances are, the engine will run worse if I throw a cheap rebuild in there. It starts withing 1/4 turn of the crankshaft when it is cold - part of the reason I am concerned because the engine is starting before it can build oil pressure. It makes plenty of power to get me over the steepest of hills. It cruises at 3300 rpm without overheating on 100+ degree days with a partially rotted radiator. It passes smog with 22ppm hydrocarbon with an allowable 350ppm. And, finally, the motor has been rebuilt once already, so it is probably at a .030 oversize and I hear factory 350 blocks don't like an .060 overbore. It is such a nice block, though, and I don't want to throw it away - 4-bolt mains and possible high nickle, high tin.

You didn't say where you are, so I don't know what kind of parts deals you can get. Here in central Florida we can get a reground crank with bearings from Central Auto Parts for $89 exchange. You'll also want to replace the oil pump while you're at it...

Before you go to all that trouble, though, your immediate problem (low pressure) may just be a symptom of a worn out oil pump. Chevy engines use a gearotor style pump. They are very reliable, but if they get too much wear between the gears and, more importantly, too much wear in the pump housing on the top and bottom of the gears, pressure drops proportionately.

You might gain some oil pressure in the short term by changing your oil to Kendall Nitro-70 racing oil. I used to run it in my stock cars because those engines were built with extra bearing clearance and the 70 weight oil helped keep the pressure up. If you live in a cold climate, though, you're going to have a problem starting that engine when the outside temp drops below about 40 degrees.

If you have the time and are willing to expend the effort, drop the pan and change the oil pump. If this solves your problem; great! If not, then the problem is worn bearings / crank journals and you will have to go for the replacement crank. You won't lose anything except your labor because if you end up having to do the crank replacement you should change the oil pump, too, and you will already have it.

There is much debate about whether a standard or high-volume pump should be used in a stock engine, but whichever you choose you can boost the pressure output by shimming the relief-valve spring. The stock spring is set to regulate at 35 lbs. If you put a .050" shim behind the spring you should get about 40 - 45 psi.

For now, I think I'm going to keep the current oil pump, and spend the extra money getting the GM bearings in .0001" increments and just match them to each journal. If any of you have a high volume oil pump you are willing to donate to a poor college student, feel free to mail it to me. I've got 21 dollars, but I need 10 for laundry.

Replacing the oil pump is still a necessity while you are doing the bearings. If the pump you have is worn out, new bearings will make little or no difference in your oil pressure...

I leaned out the idle 1/4 turn on both sides of the Q-Jet to fix the fast start up problem.
Also, I've got a Milodon stock replacement pan on my truck, but it seems to have added 1 extra quart to the sump, would I be safe using a high volume pump with that?

You may be able to chage teh bearings with the engine in the truck - a friend did it on a late 80s chevy van. youcan use plasti guage to measure the clearances.

a stock replacement oil pump is not that expensive, and no extra work at this point. my experience is pumps are cheaper at the dealer, but a few phone calls will answer that for ya.

so you hear a knocking at idle?
no metal in the oil?
Hmmm, if the clearance is big on crank (to block or rods) then you should have SOME metal in the oil -if the crank is meing eaten, then the bearings MUST be, so there should be evidence of that.

if you spun a bearing or pounded one flat, you may have to remove bearing metal from teh crank, block saddles or rods...

let us know what you find

can I just say to be careful when switchin those oils(if you do). Check contents fore ya build a sludge pond, mixing diifferent brands/grades. wasnt that a topic in hear a few weeks back?

Ok, so the plan is to measure the journals, buy the correct undersize from Clevite or Fed-Mog, get an oil pump, rear-main seal, and throw everything back together with lots of assembly lube. I have one last question, do I have to buy an oil pump priming rod, or can I just assemble the engine and crank it for a minute or two without ignition or fuel?

coming from another poor college student, go to autozone, they will rent u a primer, take it back, get ur money back, ITS FREE, so do it.