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rod bearing questions

BeloAvrgJoe

NAXJA Forum User
Location
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I have searched alot the past couple weeks and had a couple questions. In searching and in answers to previous questions I was told that the acceptable tolerance for rod bearings is 0.001"-0.003." I did also find a thread that gave a torque measurement for the connecting rod nuts, 33 lbs (Is this torque measurement correct?).

I have the oil pan off and below are the measurements from the rod bearings, starting in the front and moving to the rear

FRONT
1. 0.001"
2. 0.0015"
3. 0.002" s
4. 0.001" s
5. 0.001" s
6. 0.0015"
REAR

bearings marked 3, 4, and 5, had a noticeable scoring. I could feel depressions on the corresponding crank journals for those bearings.

With the affected journals I believe that simply replacing the bearings will only get me a little more time. The affected journals will cause quick failure in the bearings, right?

My oil pressure before beginning was not low but there was a knock I am attempting to correct. It sounded like it was in the bottom end, in the middle of the block. Could the scored bearings and affected journals be the cause a knock? And if so would replacing the bearings get rid of knock at least in the short term?

I won't get back to this thread till sometime tomorrow afternoon, I'm not a morning person especially on a Saturday, but thank you in advance for any input.
 
The clearances are good, but if there is scoring on the crank, you'll definitely want to have the crank reground, or just get a crank kit. Getting a kit will come with the bearings for the "undersize" you get, rods and mains.

The torque measurement should be listed as 33 foot-pounds (incorrect) or 33 pound-feet (correct.) The former is usually used, as far as format goes. Try to pick a torque wrench that has that value somewhere in the middle half of its range, you'll get better results. There's a reason I have something like five torque wrenches...

What compelled you to measure your rod bearing clearances, and how did you do it?

5-90
 
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Could the scored bearings and affected journals be the cause a knock? And if so would replacing the bearings get rid of knock at least in the short term?
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Maybe, or it could be wrist pin on one or more of the piston(s).

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The affected journals will cause quick failure in the bearings, right?
======================================
Without seeing it. It's a hard call. It's not the ideal condition for long bearing life.
Removal of any high spots is a must.
Doing so by hand is not for them with a fear of failings.
Most of the high spot is removed with a fine file or flat stone. TAKE YOUR TIME and get it all but do not cut in.
What looks like a small nick can often raise the surface around it.
After removing most of the high spot in this manner take strips of progress finer emery cloth and wet sand the surface by dragging the cloth over and down the surface. The last sand should be with a 1200 or finer.
Then clean, clean,clean insuring to also get the oil passageway. You do not want to leave and grit behind.
I have save more then a few cranks doing this. Never have I made "things worse" You may not get 300,000 miles off this crank. But it will most likely out live the rest of the Jeep.
 
5-90 said:
The clearances are good, but if there is scoring on the crank, you'll definitely want to have the crank reground, or just get a crank kit. Getting a kit will come with the bearings for the "undersize" you get, rods and mains.

The torque measurement should be listed as 33 foot-pounds (incorrect) or 33 pound-feet (correct.) The former is usually used, as far as format goes. Try to pick a torque wrench that has that value somewhere in the middle half of its range, you'll get better results. There's a reason I have something like five torque wrenches...

What compelled you to measure your rod bearing clearances, and how did you do it?

5-90

Basics.
Everything the original poster stated is correct.
Clearances are between .001" to .003", and the torque specs are 33 Ft-lbs.

Way Off-TOPIC
>>Both the Foot-Pound and the Pound-Foot are equals in value, though in the US, we express torque in Foot-Pounds and abreviate it to Ft-lbs<<.

Assuming these are accurate measurments across the journal, I don't see an issue as they are all between .001" and .002" You will see some scoring on a used engine, it is a fact of life. If you want to clean that up, a bit of 600-1200 grit sandpaper cut into strips some light oil and some elbow grease usually yeils good results with out affecting the journal diameter appreciably. Shines real pretty when your done also :D

True, a crank kit would fix the scoring, but is it really that severe? I can't see the piece, so I can't say for sure. If in doubt, grab a mechanic friend and have him/her look it over.

You have not mentioned what the bearing looks like, it should be a dull grey with perhaps some scoring from contamination.
If you see copper areas, or an exposed grey/copper/grey limestone like affect or severe scoring then the bearing is history. With the measurments you indicate, I would not expect to see that.

Your oil pressure is a good indication that the noise is not bearing related.
Good oil pressure relies on proper bearing clearance.

You have left us in the dark as to how many miles this mill has on it. It could be that the knock is piston slap, though that shows as a knock towards the top of the motor. Wrist pin noise should sound like that also. Most high milage "I" engines get some piston slap and it is nothing to worry about.

Ron
 
Thank you for the input. The jeep has 150k on it. I have only had it for maybe the last 2k or so. There was no knock when I got it. It was late when I finished the measurements last night and I was a little too dirty to mess with the camera.

I used plastigace to find the clearance. I took each cap off individually, put the plastigage on the bearing and then retourqed the cap to 33 lb/ft. I removed the cap again and checked the plastigage against the guide on the plastigage sleeve.

I will post a picture of one of the scored bearings (the scoring is similar across all three) and the corresponding journal. It won't be for a little while though, I've got to get some yard work done before I get all oily again. I will describe it quickly. It looks like there might have been a single piece of grit lodged between the bearing and journal. There is a scratch that goes around the bearing. I can see the copper in the scratch. On the journal I do not feel a scratch, more of a subtle depression across the surface of the journal.

I will be able to post a couple pictures after I get some other things done here around the house that my wife has been asking for.

Thank you again for the input.
 
Zuki-Ron said:
Way Off-TOPIC
>>Both the Foot-Pound and the Pound-Foot are equals in value, though in the US, we express torque in Foot-Pounds and abreviate it to Ft-lbs<<.

Not as OT as you think...

"Foot-Pounds" is a measure of work performed, and is the movement of a given "weight" over a given "distance." Anytime you read "foot-pounds," "inch-pounds," "foot-ounces," or anything similar, it correctly refers to work ALREADY PERFORMED. This is what is properly meant when the distance unit is given first.

"Pound-Feet" and similar refers to torque, which is a measurement of energy, or THE POTENTIAL TO PERFORM WORK (I only use all caps here to emphasise the point.) "Pound-feet," "pound-inches," "ounce-inches," and such all refer to torque.

Here in the States, we have actually been misusing the unit for the last sixty years or so. If you take a look at torque wrenches produced in the last few years (say, since 2001 or so,) I think you'll find that most of them are now properly labelled in "pound-feet" rather than "foot-pounds."

I know it's a sticking point, and it sounds like semantics, but it's a great way to buffalo an engineer, engineer-in-training (like meself,) or a physicist when you use the wrong unit. My using "pound-feet" to specify torque is not a regional difference or borne of my penchant for using English rather than American - it's simply using the proper unit for the task. Apparently, I simply hadn't explained it yet - so it's certainly time I did, and this provides an excellent opportunity. Thank you, Ron, for opening the door on this. :loveu:

5-90
 
Conncting rods typically wear in an egg shape. Near the center of the top bearing sleeve is usually the spot with the most wear, that's where you have to put your plasti gauge.
 
True - although the stress on the connecting rod is greatest when travelling downwards on the intake stroke, and that should result in slighly greater clearance at BOTTOM DEAD CENTRE of the bearing, or thereabouts.

However, given the striations noted, I think "egg wear" of the bearing shell is less of an issue. The striations will reduce the contact area of the bearing, and reduce the ability of oil under pressure to flush out the bearing contact area, which indicates replacement/regrinding. I'd be interested in knowing if the original poster uses Fram filters - or some other sort of cheapass filter - and has a history of doing so?

Crankshaft bearings are not something I'd generally check right off, or as a PM procedure - but if I have checked other related issues and found nothing. I'm just cranky that way. I find I check bottom end bearings by Hobson's choice, and only with DAMN GOOD cause...

5-90
 
5-90 said:
Not as OT as you think...

"Foot-Pounds" is a measure of work performed, and is the movement of a given "weight" over a given "distance." Anytime you read "foot-pounds," "inch-pounds," "foot-ounces," or anything similar, it correctly refers to work ALREADY PERFORMED. This is what is properly meant when the distance unit is given first.

"Pound-Feet" and similar refers to torque, which is a measurement of energy, or THE POTENTIAL TO PERFORM WORK (I only use all caps here to emphasise the point.) "Pound-feet," "pound-inches," "ounce-inches," and such all refer to torque.

Here in the States, we have actually been misusing the unit for the last sixty years or so. If you take a look at torque wrenches produced in the last few years (say, since 2001 or so,) I think you'll find that most of them are now properly labelled in "pound-feet" rather than "foot-pounds."

I know it's a sticking point, and it sounds like semantics, but it's a great way to buffalo an engineer, engineer-in-training (like meself,) or a physicist when you use the wrong unit. My using "pound-feet" to specify torque is not a regional difference or borne of my penchant for using English rather than American - it's simply using the proper unit for the task. Apparently, I simply hadn't explained it yet - so it's certainly time I did, and this provides an excellent opportunity. Thank you, Ron, for opening the door on this. :loveu:

5-90

Seriously OT at this point, and you have decided to hyjack the thread to prove your point. This thread is not about you, your career moves or what you believe to be the correct term. It is about rod bearing wear and this fellows difficulties.
Let's stick to the subject, shall we?
Ron
 
Zuki-Ron said:
Seriously OT at this point, and you have decided to hyjack the thread to prove your point. This thread is not about you, your career moves or what you believe to be the correct term. It is about rod bearing wear and this fellows difficulties.
Let's stick to the subject, shall we?
Ron

All I was after was trying to correct the use of terms - and I gave some of my background to illustrate education on the subject.

It's not "what I believe to be the correct term" - it simply "is the correct term." The US industry is changing to reflect that, while the ISO and industry overseas has been using the correct terms for years. Since we're in a state of flux with terminology, I'm just trying to help us stay right on - or maybe a little bit ahead of - the curve.

(Back to task...)

Given that there are striations on the bearings and probably on the crankpins, it would be helpful to us if you could post pictures of both. Also, the fact of the bearings showing consistent striations on three pins, and the crankpins showing striations as well, makes me wonder a bit. I'd have to see this thing in person, but it's not something I'd be comfortable servicing unless it was on the bench, and it may not be saveable without grinding anyhow - sanding can only remove so much, honestly, and your crank may be beyond that. I can see sanding the crankpins working if the straitions are not deep (visible, perhaps, but not tactile,) but if you can feel them, they're probably deeper than you think. Not to belittle badron (I don't know him personally, but he makes a good noise. I've heard from him before...) but I also don't know what your experience level happens to be, or how familiar you are with what happens when a tool starts to cut metal (and a crankshaft is not something I'd want someone to practise on, if I intended to reuse it. Too critical.)

"Bearing scraping" is also hideously easy to screw up - I haven't even tried it yet. I'd sooner scrape the bed of a Bridgeport flat than try to scrape bearings to profile right now - due to a lack of experience at either (but the grey iron of a Bridgeport bed it rather harder to cut than the Babbitt metal of a bearing, which makes it harder to screw up.)

Honestly, if you have the wherewithal to post pics, that would help greatly. You can only describe something so much before you end up losing something in the translation - and that just will not do for diagnosis. Help us help you...

5-90
 
You've got a bottom end knock on an engine with 150k on the clock. Save all the measuring and rebuild headaches and just swap in a junkyard motor. Jeep 4.0 engines are a dime a dozen at the boneyard, so they go pretty cheap. Easier to just drop a whole new engine than to remove, rebuild, and reinstall the original IMO.
 
Rob bearings are cheap ... it's worth a try. Buy a set of bearings, get some crocus cloth (which is super-fine grit emery cloth, mentioned in automotive shop manuals specifically for polishing crank journals, give the journals a buffing with the aforementioned crocus cloth, and slap it together. It cannot be any worse than it was.

If your plastigage measurements are accurate, though, most likely the knock wasn't the rod bearings. Could be a wrist pin, or it could be a cracked flex plate if you have an automatic.
 
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