One rod bearing too tight. What to do?
 

I'm replacing the rings on a '80's 4 cyl. (not M.B.) and thought I might as well replace the rod bearings while it was apart. Bad idea. When I tightened down the bolts on the last rod I was doing (#3), I couldn't turn the crank with a wrench on the pully bolt. When I loosen that rod it turns fine. Plastigage shows a clearance of .0015" while the other rods showed .002" to .003" which shouldn't make much of a differance. Specs are .0008" to .002" with maximum of .0031" I tried using an old bearing on the lower half with no change. Should I also put back the upper half as well? I could turn the engine over be hand before taking it apart.

There must be something else wrong.

Are you sure you have the tabs in the correct position? Look on the back of the bearings - are they the same part number as the others? Are you sure you've not crushed anything behind one of the bearing inserts?

Yes, everything is in the correct position. If I tighten the rod bolts to 10 ft lbs it's fine, but any more and up to the 36 ft lb spec and it won't turn so it can't be too much out of whack. Tomorrow I'll caliper the rod journal and the bearing thickness to compare the specs.

Checking bearings
 

Install the bearings into the connecting rod without installing it on the crankshaft and snug down the rod cap fasteners. Use an inside mic to measure the inside diameter in about 6-8 different places. This way you will get the ID of the bearing bore and you will also be able to tell if the bearing is out of round when installed on the connecting rod.

You could also swap those bearings to another rod to see if you have the same problem after torquing the rod caps down.

You could put the old bearing back in the same rod and see what happens. (both halves)

Good luck...

Did you install standard or undersized bearings?

Sixto
93 300SD

They are standard size bearings.

The bearing cap and rod are a matched set. That is, they're bored as an assy to line up perfectly round. If, by some chance, you mixed up the cap from a different rod, this could pinch the bearing against the crank bearing journal.

If you think this might have happened, take a close look at the machine marks on the bearing cap end and rod when assembled. They should match up perfectly. In other words, the scratch marks in the bore from the cap end should follow perfectly to those on the rod side.

If you're sure nothing got mixed up and everything is clear of debris, you may want to try another set of bearings. They could be bent or defective. Shouldn't be too much $ for a 4cyl 80's car.. Especially since it's not a Benz. ;)

Good luck.

Or the bearing cap went on back to front.

Can you fit a bearing set from another rod to see if that has proper clearance? You might be able to return that bearing set as defective.

Sixto
93 300SD

It's not unheard of for a bearing set to have one odd-sized bearing half. Check the markings. If one is marked wrong, you know the problem.

Otherwise, as sixto implies, you might have a bad bearing half. Plastigauge won't necessarily show a bad half or even one of the wrong size, as the binding won't be evenly distributed. Definitely try another set in this rod to see what it does. Then you'll know.

I swapped bearing caps with another cylinder and the crank turns freely now.

You swapped bearings, or you swapped end caps?

Of course, you should not run the engine with mismatched end caps. That's asking for catastrophic engine failure.

You had better read this post again. He is right on the money. You CAN'T mix and match rod caps. YOU MUST check the rods and caps and re mate them to each other. As later said in other posts..... You are inviting catastrophy

Glad to hear you found the problem. I'd still recommend getting a new set of bearings though, since the ones you had torqued down on the mismatched rod and cap set is probably now distorted. Doesn't take much, as the surface is made of a soft lead alloy metal.