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Unread 02-09-2010, 02:37 AM   #1
hasher09
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1991 YJ Wrangler 
 
Join Date: May 2009
Location: santa rita,guam
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4.0 main bearing replacement ?s

these are legitimate questions here, and i can only hope there are some legitimate answers for me and other YJ owners. i drive a 91 YJ 4.0 5spd. 150 k on the odo. over the last couple of months i have been experiencing low oil pressure. just like a lot of other threads. if the jeep has been sitting for a while, i start the jeep and oil psi is around 45. but , when the jeep gets to temp it sits around 5 at idle with a small rise to around 15 psi. even at 3 k rpm. so i put in a new sender( same results ) installed mech. gage( same results) so as much as i dont want to admit it i have to replace the main bearings, gonna swap oil pump as well. also gage is " newer" ( sunpro )

so the questions are :
1. can i replace the main bearings without removing tranny and crankshaft?
2. if so , do i have to remove the timing chain to be able to lower crankshaft a lil ?
3. will a plastigage tell me how far out of tolerance my bearings are?
3. what size replacement bearings should i buy? some are .010 over and so on...
thanks guys -

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Unread 02-09-2010, 06:03 AM   #2
jsawduste
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2006 YJ Wrangler 
 
Join Date: Jul 2006
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Quote:
Originally Posted by hasher09 View Post
these are legitimate questions here, and i can only hope there are some legitimate answers for me and other YJ owners. i drive a 91 YJ 4.0 5spd. 150 k on the odo. over the last couple of months i have been experiencing low oil pressure. just like a lot of other threads. if the jeep has been sitting for a while, i start the jeep and oil psi is around 45. but , when the jeep gets to temp it sits around 5 at idle with a small rise to around 15 psi. even at 3 k rpm. so i put in a new sender( same results ) installed mech. gage( same results) so as much as i dont want to admit it i have to replace the main bearings, gonna swap oil pump as well. also gage is " newer" ( sunpro )

so the questions are :
1. can i replace the main bearings without removing tranny and crankshaft? NO
2. if so , do i have to remove the timing chain to be able to lower crankshaft a lil ? YES
3. will a plastigage tell me how far out of tolerance my bearings are? NO
3. what size replacement bearings should i buy? some are .010 over and so on...Have the crank journal checked for taper, out of roundness. Will likely need to be reground
thanks guys -
There is no way to do this job right without removing the crank
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Unread 02-09-2010, 07:14 AM   #3
Bastidjeep
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Quote:
Originally Posted by jsawduste View Post
There is no way to do this job right without removing the crank
I agree as far as doing the job 100%. You can replace them in frame though and without removing the trans or the timing chain. Cost wise you are better off in the long run to either rebuild or replace the engine. You'll spend at least $200 on mains and rods, oil pump, pump pickup and gaskets.
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Unread 02-09-2010, 07:43 AM   #4
jsawduste
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Quote:
Originally Posted by Bastidjeep View Post
I agree as far as doing the job 100%. You can replace them in frame though and without removing the trans or the timing chain. Cost wise you are better off in the long run to either rebuild or replace the engine. You'll spend at least $200 on mains and rods, oil pump, pump pickup and gaskets.
How are you going to remove the crank without removing the timing chain ??? The drive sprocket is on the crank snout.

If he has a manual trans getting the crank out without removing the clutch/pilot would be a trick.
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Unread 02-09-2010, 07:46 AM   #5
Bastidjeep
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Quote:
Originally Posted by jsawduste View Post
How are you going to remove the crank without removing the timing chain ??? The drive sprocket is on the crank snout.

If he has a manual trans getting the crank out without removing the clutch/pilot would be a trick.
You don't need to remove the crank to install inserts. Loosen yes, remove no.
Sorry, I should have been more clear.
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Unread 02-09-2010, 08:08 AM   #6
Jeeeeeeeeeeeeep
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Quote:
Originally Posted by jsawduste View Post
There is no way to do this job right without removing the crank
I agree. I guess anything is possible, but I have never heard of anybody doing main bearings without removing the crank , and while it is out bring the crank to the machine shop. They will tell you if the crank needs to be turned, if it does they then will tell you how much they turned it, .010, .020 etc. At that point you might as well rebuild the whole motor. You'd be a lot happier in the long run.
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Unread 02-09-2010, 08:22 AM   #7
everydayXJ
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With the dissenting opinion:

Have you done a cylinder leak down test? Not a compression test, an actual leakdown test? If you can do that right, and your top end is still in good shape then I'd go ahead with rolling in new bearings. It's done a hundred times a day in the diesel engine world, your 4.0 is just a tiny sized version with slightly cleaner oil. The timing chain will have enough slack for you to allow just a touch of droop on the crank.

All that said, there are plenty of reasons this wouldn't be worth it.
Leaking rings, leaking valves, scored crank journals, bad cam lobes, etc..

But you owe it to yourself to find out the real condition of your engine before you make this decision.

Good luck.
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Unread 02-09-2010, 11:49 AM   #8
DaveinHouston
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Guys I'm a newbie to this site but I've been rebuilding engines for years. I'm sorry but If I'm going to go to the trouble to replace the main bearings (what about the rod and camshaft bearings) I'm going to pull the engine and do an overhaul. That's of course after having checked the engine out well and satisfied myself that is indeed the bearings and not the oil pump.
As to the question about oversize main bearings they are made to fit a crankshaft that has been reground to that dimension, not to make up for wear on the bearing journal.
And for plastigauge I've only ever used it on new bearing installations to verify that the clearance between the bearing and journal is within tolerance. I'm a big believer in the use of plastigauge but not to check for wear.
Just my two cents. Good luck on the engine.
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Unread 02-09-2010, 12:42 PM   #9
Jeeeeeeeeeeeeep
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Quote:
Originally Posted by DaveinHouston View Post
Guys I'm a newbie to this site but I've been rebuilding engines for years. I'm sorry but If I'm going to go to the trouble to replace the main bearings (what about the rod and camshaft bearings) I'm going to pull the engine and do an overhaul. That's of course after having checked the engine out well and satisfied myself that is indeed the bearings and not the oil pump.
As to the question about oversize main bearings they are made to fit a crankshaft that has been reground to that dimension, not to make up for wear on the bearing journal.
And for plastigauge I've only ever used it on new bearing installations to verify that the clearance between the bearing and journal is within tolerance. I'm a big believer in the use of plastigauge but not to check for wear.
Just my two cents. Good luck on the engine.
Exactly, this is what I tried to say although I didn't do half as good of a job trying to explain it.
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Unread 02-09-2010, 12:44 PM   #10
Jeeeeeeeeeeeeep
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Quote:
Originally Posted by DaveinHouston View Post
Guys I'm a newbie to this site but I've been rebuilding engines for years. I'm sorry but If I'm going to go to the trouble to replace the main bearings (what about the rod and camshaft bearings) I'm going to pull the engine and do an overhaul. That's of course after having checked the engine out well and satisfied myself that is indeed the bearings and not the oil pump.
As to the question about oversize main bearings they are made to fit a crankshaft that has been reground to that dimension, not to make up for wear on the bearing journal.
And for plastigauge I've only ever used it on new bearing installations to verify that the clearance between the bearing and journal is within tolerance. I'm a big believer in the use of plastigauge but not to check for wear.
Just my two cents. Good luck on the engine.
Double post, sorry.
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Last edited by Jeeeeeeeeeeeeep; 02-09-2010 at 12:46 PM.. Reason: sorry, double post
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Unread 02-09-2010, 01:15 PM   #11
4.7stroker
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Quote:
Originally Posted by hasher09 View Post
these are legitimate questions here, and i can only hope there are some legitimate answers for me and other YJ owners. i drive a 91 YJ 4.0 5spd. 150 k on the odo. over the last couple of months i have been experiencing low oil pressure. just like a lot of other threads. if the jeep has been sitting for a while, i start the jeep and oil psi is around 45. but , when the jeep gets to temp it sits around 5 at idle with a small rise to around 15 psi. even at 3 k rpm. so i put in a new sender( same results ) installed mech. gage( same results) so as much as i dont want to admit it i have to replace the main bearings, gonna swap oil pump as well. also gage is " newer" ( sunpro )

so the questions are :
1. can i replace the main bearings without removing tranny and crankshaft?
2. if so , do i have to remove the timing chain to be able to lower crankshaft a lil ?
3. will a plastigage tell me how far out of tolerance my bearings are?
3. what size replacement bearings should i buy? some are .010 over and so on...
thanks guys -
I will answer these questions as asked.
1, Yes you can, it may be somewhat difficult but it can be done.
2, No there will be enough slack.
3,Yes it will but remove all oil from the mating surfaces where the plasti gage is to be used.
4, Use the same size bearings that you take out and this is when you will use the plastigage.

Now weather this is the right action to take is up to you, as long as the top end is good I probably would take the risk of installing new bearings and a new oil pump if time was tight and I was positive nothing else was wrong with the engine.
Do not for get to take the new oil pump apart, clean and fill with something like STP or engine assy lube before you install it.

Dwayne
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Unread 02-10-2010, 03:28 AM   #12
hasher09
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engine loping

this is the kind of feed back i need. i am in the works for a quick fix, but i dont want to halfass my engine.another thing i notice besides the low oil psi is a kind of loping sound. it doesnt happen until im in 4th gear cruising around 45 50. and the engine doesnt exactly have a nice smooth rumble. its like a wha - wha -wha - wha -wha -wha . making me think the crankshaft is most deffinetely warped. i do not have the tools nor the ability to do this work myself. i think i am going to look for a new engine.the rest of the jeep is in too good of condition to just sell or scrap. anyone have any good links on replacement engines?
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Unread 02-10-2010, 05:26 AM   #13
dodger889
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Are you sure that the engine is causing the sound. I have a rear end that is going that makes that sound. If that sounds like a plane drone sound of an old prop engines. You might start looking other than the engine for your problem a vibrations are sometimes hard to figure out where they are coming from. If the bottom end was a minor problem it would be more of a knocking sound.
Now to address the oil problem now oil pumps do wear out they have a plate that with wear will let oil to bypass around the oil gears.So to replace them are not that hard just drop the oil pan and replace.
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For those who have not figured this out I'm old school but can handle the new too.
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Unread 02-10-2010, 05:59 AM   #14
Larry93
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If you really want to ghetto fix the problem, go to summitracing and buy the high volume oil pump. You will need to massage the oil pan a bit to clear it but you will no longer have an oil pressure issue. What you will now have is an oil consumption problem. Also your engine will develop a apetite for distributor gears.

With mobil 1 0W-30 I have over 50 psi at idle hot.
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Unread 02-10-2010, 07:03 AM   #15
Bastidjeep
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My suggestion is to do a leak down test. This will tell whether or not you have good valve seal as well as the condition of your rings. A compression test will not tell you this. It's more of a quick generic type of test. If you decide to do both tests and it passes, then You could likely get away with installing new inserts.
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