Replacing Rod Bearings

[NWFMB]

I have not pulled the rod yet, but can I assume that the 3 different rod bearings available - 52.00mm, 51.50mm and 51.25mm - are dependent on the wear? In other words after I pull it and mic it I would compinsate with a different size bearing.

Thanks

[Brian Carlton]

The different bearing sizes are produced to allow a machine shop to grind the crank journals undersize, if they are in poor condition.

You won't have a selection of bearing sizes available to you, if the crank is not reground. You will have to utilize the stock (maximum) size. If the crank is not within specs for these bearings, it must be reground for a larger size.

[P.E.Haiges]

NWFMB,

At one time .001" and .002" undersize bearings were available for American cars to compensate for wear. But the trouble with that is the wear usually causes the journal to be egg shaped. I have also known mechanics to put shims behind bearings to compensate for wear.

Brain is correct, to use the bearings you specified, the crankshaft has to be reground.

P E H

[NWFMB]

Thanks for the clarification, lets hope they are still in spec, as it is on #1 cylinder, therefore I do not intend to pull the whole engine, but just replace the one and go from there.

Thanks again.

[P.E.Haiges]

NWFMB,

Most important is the condition of the crankshaft journal. If it is rough, it will chew up the bearing and the repair won't last long. If the crankshaft journal is rough or the clearance is above specification, the journal must be reground. MB turbo cranks are nitrided so the journal might have to be nitrided again is you want to have a perfect repair.

Perhaps a replacement crankshaft is a better choice.

P E H

[Pete Burton]

1/16" of play sounds huge, but taking it out will tell for sure. Check journal with a mic 90 deg apart. You might get lucky, those cranks are very hard. I'm sure you'll check the clearance at the wrist pin. Good luck.

[P.E.Haiges]

NWFMB,

I agree that 1/16" is a huge amount of movement.

If you replace the rod bearing, check the clearance with plastigage and you will have an idea of how much clearance there is. You should put in 2 pieces pf plasticgage 90 degrees apart.

P E H

[NWFMB]

I have the bottom of the rod pulled off, and am ready to pull the head tomorrow. The bearings are definitly shot, but so far I can not see any damage to the rod. Will know for sure when I have it out. The journal seems smooth (haven't mic'd it yet), yet it seems that it might be out of round a little (the direction of movement). I will check it more closely later.

I plan to use plasticguage to double check ( have not used it before, but looks like a great product.

1/16th maybe a tad more than what it is, but it was a loud knock that was getting worse while troubleshotting the engine, but never drove full power or highway speeds since I noticed the knock, if I had it wouldn't have been long and there would be a hole in the case I am sure...

Any shortcuts on getting the bearing pin out for the chain guide if you don't have an impact puller that will work on it. I will see if I can borrow one from someone.

Thanks

[Pete Burton]

You can pull that pin by inserting a bolt into it and a number of flat washers. The ID of the flat washers next to the pin needs to be larger than the pin OD. After you've moved the pin 1/4" or more you can remove all but 1 washer, slip a small open end in and pry - (you can also use a thin screwdriver as a fulcrum)

[t walgamuth]

i feel like i must have come in in themiddle of aconversation here, but it soounds to me like you need to pull the engine out and go through everytthing. i cant imagine how one rod bearing could be bad and everything eelse is ok.. also there ust be material all through the engine from the bearing breaking down.

[P.E.Haiges]

NWFMB,

Why are you pulling the head off? You might have to pull the upper oil pan to get at the other bearings, but you shouldn't have to pull the head off.But its still possible the head won't have to come off unless you want to replace the rings or do a valve job.

Its possible you may have to remove the engine to get the upper oil pan off. If you have to R&R the crankshaft, engine must be removed.

P E H