Big-end bearings - why replace all?
 

I took off the left bank of cylinders on my '85 Carrera to search for the source of a knocking sound, and it appears that the no.3 big-end bearing is faulty. The conrod doesn't move up and down freely, and it has way to much play sideways. So I suspect the bearing.

The plan is to replace all 6 small-end bearings with their nuts & bolts, because that's the right thing to do. I just wonder why it's the right thing to do, why not only replace the faulty one? Or will one faulty big-end affect the others?

Thanks

well, those other bearings may not be causing a problem now, but since you've got the thing apart it is much cheaper and easier to replace them now (not too expensive) than to have to pull the thing apart again in 5, 10, or 15K miles.

JW showed me that bearings trap/capture particles. If you look closely at a used bearing, you will find rocks and things embedded in them.

If you want my advice, I would tell you to split the case, replace main and rod bearings and intermediate shaft bearings, and MEASURE ALL THE PARTS. You'll be disappointed if you don't do this and your engine still makes a noise.

Patronus,

You may find that #3 crank throw to be scored and way beyond a simple bearing replacement 'fix' for your knock! Worn rod bearings usually make that rod VERY EASY to move up and down ... until the bearing spins, cuts into the crank with a bit of 'grit' like a polishing wheel ... crank turns blue, etc.

Gentlemen, I just removed the #3 con rod and what I found was shocking. The big end bearing has almost disintegrated, it is in pieces. The con rod itself appears to be slightly pitted, very shallow but visible. I can't really see damage on the crank, it has a few thin lines but it can possibly be dirt, or bearing residue. It's not pitted and not blue.

Suppose the crank's surface is indeed damaged, what stand me to do? Can it be re-surfaced or must it be ground to a different spec?

As far as the con rod is concerned, I assume it will have to be machined to accept an over-size bearing?

Thanks for the advice so far, I will measure & replace everything as far as I go.