main bearings

chuck619120

NAXJA Member
NAXJA Member
Any one ever replaced main and rod bearings with the motor still in the jeep? I have one knocking, thinking about doing main bearings, rod bearings, oil pump, rear main seal, and timing chain with motor still in jeep. Thoughts?
 

Talyn

New member
It can be done. I don't know if its worth while because the crank will have wear as well. Worse case you will be out a set of bearings and a bunch of time.
 

jerms234

New member
It can be done, you may need to engineer a tool that fits into the oil channel in the crank to help remove the upper main bearing halves. Be sure to use lube on the new rms, it will tear otherwise. However I don't believe this is worth the effort. Let me explain.

If the knock is indeed a bearing issue, crank journal scoring is likely. To correct this scoring the crank journals should be resurfaced. I've seen two 4.0s with knocks, one was a wrist pin knock, the other was a broken piston skirt. I have not seen a 4.0 knock with a bearing issue, I'm by no means a 4.0 expert either. I'd highly recommend pulling the motor, and doing a full rebuild. It's the only way to be sure you have completely resolved the issue.

My .02$

Jeremy
 

cody4359

New member
bought a parts rig and can't bring myself to tear it apart since it's so clean. here was my knock. bore looks fine, mic'd the top bit not bottom. new piston, rod bearings and mains on the way. Doing it with the motor in.


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chuck619120

NAXJA Member
NAXJA Member
Thanks for replies

It can be done. I don't know if its worth while because the crank will have wear as well. Worse case you will be out a set of bearings and a bunch of time.
Didn't think about the worn crank, hopefully not too bad yet.


It can be done, you may need to engineer a tool that fits into the oil channel in the crank to help remove the upper main bearing halves. Be sure to use lube on the new rms, it will tear otherwise. However I don't believe this is worth the effort. Let me explain.

If the knock is indeed a bearing issue, crank journal scoring is likely. To correct this scoring the crank journals should be resurfaced. I've seen two 4.0s with knocks, one was a wrist pin knock, the other was a broken piston skirt. I have not seen a 4.0 knock with a bearing issue, I'm by no means a 4.0 expert either. I'd highly recommend pulling the motor, and doing a full rebuild. It's the only way to be sure you have completely resolved the issue.

My .02$

Jeremy
If I was to pull motor, I would prolly just buy a long block replacement. If I can make it last another six months by doing this, I will do a swap next summer.


bought a parts rig and can't bring myself to tear it apart since it's so clean. here was my knock. bore looks fine, mic'd the top bit not bottom. new piston, rod bearings and mains on the way. Doing it with the motor in.


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Did you see shavings in the oil? I have never found shavings in the oil. It doesn't knock all the time. I start it and no knock. When temp reaches 100 degrees it starts with a knock and then goes away after reaching full operating temp.
 

cody4359

New member
oil was pitch black. I bought it for the body harness but now it's an experiment. I poured some kerosene in to clean out the sludge and let it idle, I don't think that helped anything. the cylinder that was buggered up still had a bunch of sludge buildup so I think It got starved for oil?

that pistons rod bearings were good on the lower half and had more wear on the upper. I'm just going to use the BACK of my emery cloth to lightly clean up the crank. it's not perfect but not FUBAR'd either. if it works, great if not I'm only out about $100 and my time.

this thing was noisy. my comanche has a slight knock/tick, I was thinking of doing bearings on it but after this I'm starting to lean twoards cracked piston skirt being my noise so I'll run it til it's bad and easier to diagnose

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old_man

NAXJA Member
NAXJA Member
I have done crank/rod bearing replacements on several vehicles over the years, but never in a 4.0L. I wouldn't hesitate to do it again. No you don't get as much benefit as a full grid and replacement but every engine I have done it to lasted well over 50k before I got rid of them and none failed.

Everybody would always like to do things the "right" way but my 50 years as an engineer has finally taught me that sometimes less is good enough, especially when funds are tight.
 

lazyxj

NAXJA Member #1310
NAXJA Member
Changing the bearings in frame isn't much more involved than changing the RMS.
The worse part is constant drip of oil in your face, even after it sits overnight.
Wear your safety glasses...and an old tee shirt.
 
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