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'99 Trooper Rod Knock

Can I fix a bad rod bearing by dropping the oil pan and reconditioning the crank journal in question by hand with emery cloth (assuming the journal’s not too bad) and installing a new bearing? And a new rod if it’s damaged?

Hey, all you pros and semi-pros out there, I know this is theoretically possible, but I’m wondering whether such polishing treatment is normally successful, or whether even a slightly bad journal must be reconditioned by machine. Also, would there be any problem pulling the piston & rod out the bottom of the engine? Or would it have to go out the top? I’ve never had to fix a rod bearing on a 4-wheeler before, so the exact practicality of this approach is beyond my experience. Thanks.

I think that the crank journal may need more reconditioning than what can be done with emery cloth. Years ago, there used to be technicians that had a machine and would go from shop to shop regrinding journals with this portable machine with the engine still in the car so that the shop could fit a new, undersized rod bearing. I’ve seen this done. The cylinder head, though, was removed, the ridge at the top removed with a ridge reamer and the piston and rod pushed out through the top of the engine. I don’t know if this procedure is even done today.

The rod and piston cannot come out the bottom. It has to go out the top so this means removal of the cylinder head and ridge reaming.

As to whether a new rod bearing will last more than a few weeks; that all depends on mileage, the reason for the rod bearing failure, the amount of egg or taper in the rod journal, etc.

While I was in the States and my wife was in Switz. she bought us a 4 cyl BMW (not new) we put many thousands of miles on it,one day it developed a rod knock,I inspected, it was clear the pan had been removed, I located the rod that had the bad bearing,the rod journal looked perfect no damage at all. I put a new bearing in and the dealer I worked for exported the car.

What we think happened is the bearing failed previously and was repaired just as I did,even though the journal looked perfect to the eye it was still bad enough to cause bearing failure. I don’t have much faith in just cleaning up the journal with emery cloth.

OK. Well, fellas, you sure know how to shoot down one’s best hopes. I guess the answer is a replacement bearing could last either through the afternoon or “many thousands of miles,” and you don’t know which until it fails. It’s a total crapshoot. So much for that idea. But I do have a question for you, oldschool. How did you locate the bad rod? Did you just start pulling caps until you found a bad-looking bearing? Or is there some method of divination that reveals the culprit without trial-and-error disassembly? And I guess you didn’t mic the journal to see how it was out of tolerance, or did you? That would be very interesting to know.

And as you guys indicate, in a case like that of oldschool’s BMW, in which the journal was presumably the original problem, touching up the journal surface with emery cloth to remove any fine scratches made by the bad bearing would be completely missing the point – an exercise in futility. I would wager that most cases are just like that.

I’ve always found it interesting that a rod bearing can make that atrocious pounding noise though it has only a coupla thousandths to move, if that. I’d like to see or hear an explanation of exactly what movement or force produces that gawdawful sound.

Thanks for all your good advice. I appreciate it.

A rod that is pounding loudly will likely have more than a couple of thousandths of clearance in it. Odds are the overlay is worn off the bearing rather than the journal being worn; unless it was one of those run out of oil things.

It is possible to simply install a set of bearings and go many miles but a lot depends on the journal surface and diameter. Every time I’ve done something like this I’ve always miked the journal. A half thousandth out of round is survivable but if you get into that .002-.003 out of round then they won’t last.
Technically all of the bearings, including mains, should be replaced because worn mains cause low oil pressure which then affect the rod bearings also.

If you want to know which cylinder it is knocking, remove one plug wire and start it up briefly. If the noise remains, shut it off, reinstall the plug wire, and repeat on another cylinder. When the noise subsides quite a bit you know you’ve found the bad one; or at least the really bad one.

Let’s see . . . I’m calculating the red-neck solution . . . replace the bearing and run 20w-50 instead of the usual 10w-40 or 5w-30 or whatever. Ya git hi-er awl preshure an’ more o’ that there film strinth. Heck, ah can throw in a kwart o’ 75w-90 fer good mayzhure.

That diagnosis method is a really slick trick, ok4450. But the last part of your last sentence is very discouraging.

Thanks again, fellas.

I use the pull the sparkplug wire trick to get my best guess (there were only four to choose from)pulling the wire didn’t eliminate the noise completely but the change was obvious,zeroed right in on it. I think the car went to Lybia (it was the 80’s they deserved it) No I didn’t mic the journal,either the bearing replacement was going to stop the noise (at least for a while) or the car would be exported as scrap (non-running engine) there was never a possibility of a full rebuild.

Since I worked for a BMW Dealer in Switzerland I had easy access to both the export program and a new car (got my 320i 6cyl).