'91 3.1 Sunbird - Replace connecting rod bearings?

You might be missing something...

The main bearings should also be replaced. I believe this is the main reason you are having oil pressure problems. Likely it has very little to do with the oil pump.

Changing only the rod bearings won't help much.

Have you tried running thicker oil? Try 15-40 or even 20-50. 200,000 miles is a lot to only replace a few bearings. And an expensive oil pump.

Thanks,

No, I haven't tried thicker oil. I was wondering though, is it still ok to replace bearings without having to remove the crank, and have it turned, or otherwise prepared? I'd like to think I can simply remove the bearing caps and slide in some new bearings.

Mark.

You can do this, but the problem is that if the crank and rod journals are worn or damaged the new bearings won't last long. At a minimum you will need to mic the rod journals, but there's no way to check the mains without removing the crank, which means pulling the motor to get at the flexplat bolts.

JazzMan

Shoot. Didn't want to do that. Currently, there's no audible sign of a problem (no knocking, etc.), and there's no Engine vibration or anything. I was just going to do this to help oil pressure, and kinda for preventative maintenance. So, if I do decide to just change the bearings, could I be doing something worse to the engine by doing so? Know what I mean?... I'm thinking I can only make things a bit better, but if there's a chance I can end up doing more damage, I won't touch it.

Thanks, Mark.

Plasti-gage can be used to check the mains in situ. The rods too, but a mic is quicker.

Chas Hurst

"Mark" wrote

I don't see a problem with this. If you don't see any bad scoring of the rod journals...i.e: scoring that you could feel with you fingernail.....I would just slide in some new bearings. Even if you have some wear on the rod journals...a new set of bearings will reduce your clearances somewhat. It's a bit of a patch job, but if you are constrained by budget limitations...it may give you a bit more time.

I've installed .001" undersized bearings on a couple of engines of my own that just needed to have some clearance reduced.....it worked just fine. Bearings and cranks are a lot more hardy then people give them credit for. Of course, I'm mainly speaking about what you can do "for your own vehicle". I would not do this kind of repair at the dealership, as there is no way to warranty this kind of repair. But it's amazing what you "can" do with automotive patches.

Ian

If there's currently no obvious problems and you are doing it as a preventative maintenance, then it's very do-able and should have no down side (providing you are careful and you make sure to use plenty of assembly lube during reinstallation). You can check the fit of the mains using plastigauge (before lubing the lower shell) to see if it's within specs. As JazzMan pointed out, the operation usually doesn't last if it's a repair situation as the crankshaft is usually damaged in these situations.

Okay great!

Thanks Ian, and to all others who replied.

Mark.

Before going thru that. I'd make sure the engine idle speed was right in drive and check the pressure with a mehanical gauge in park. If you don't have any knocks, I'd just check the mains with plastigage. I have never done an in-car bearing change. Getting the pan off may be the biggest challenge. And what would be the best tool for unseatng the bearings without scratching the crank?

"TOLYN9" wrote

No special tool needed. You just push the upper half of the main bearing at the non tang end and it will roll right out. If you remove all the main bearing caps, the crank will actually be hanging down just a smidge, makes it quite easy to roll out and roll in the bearing shells.

Ian

In my lmited experience, Some need some coaxing out of the saddles even with the crank out. I tap them around with a screw driver. I'd be inclinded to use a softer tool in the presence of the crank is all I am saying.

Thanks guys. I figured the bearings could "roll out" with the caps off, and the crank just hanging down a "smidge". I was going to persuade them with a small piece of hard-wood (like the size of a popsicle stick).

Mark.

What does it mean ti "mic" a crankshaft? Thanks.

Use a micrometer or other precision measuring device to measure the diameters and shapes of the parts of the crank that ride in bearings, such as the main block/crank bearings, or have connectiong rods/bearings riding on them. These bearing surfaces can wear down, and wear out of round. "Mic" is short for micrometer.

JazzMan

What oil are you running in it?? I hope not 5w-30. I would run 10-30 or 15-40 and one container of STP oil additive, or Hi-Tach, or Lucas oil stabilizer.. If you have no knocking or tapping...your bearings are fine...you just have some wear causing increased oil journal clearance that is totally normal for a high mileage engine.. I wouldn't even bother with a oil pump. I would get the thicker oil in there then get it good and hot and hook up a mechanical guage to test the actual accurate oil pressure when the oil is its thinnest. Then see if it is within specs....I bet it will be....I have seen many engines that only have to have a min of 6-8 psi at 1000rpm with thin hot oil.....