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Unread Yesterday, 09:09 PM   #1
Chstock
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1984 CJ7 
 
Join Date: Jun 2013
Location: Newberg, Oregon
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Finally got my cj on the road! And got a rod knock

Finally got the jeep on the road and legal, rod knock literally a week later. Is my only road from here an engine swap? Crate or otherwise?

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Unread Yesterday, 09:11 PM   #2
kirbot
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Are you sure it's rod knock?
Piston slap sounds similar, and it's VERY common with these engines.
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Unread Yesterday, 09:25 PM   #3
Chstock
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Quote:
Originally Posted by kirbot
Are you sure it's rod knock? Piston slap sounds similar, and it's VERY common with these engines.
the mechanic that was tuning the carb told me it was so as far as I know yes. Is there an easy way to tell the difference?
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Unread Yesterday, 09:43 PM   #4
kirbot
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Quote:
Originally Posted by Chstock View Post
the mechanic that was tuning the carb told me it was so as far as I know yes. Is there an easy way to tell the difference?
Piston slap will usually be louder when you first start the engine, and then get very quiet or disappear completely after 5-10 seconds. It might take longer for some engines, but that's how mine acts.

When I had a car with bad rob bearings, they would sounds worse under hard acceleration/heavy load. I don't think that should affect piston slap very much.

How many miles are on the engine?

If all else fails, it's not that bad of a job to drop the oil pan and check the rod bearingsd
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Unread Yesterday, 09:45 PM   #5
Chstock
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Quote:
Originally Posted by kirbot
Piston slap will usually be louder when you first start the engine, and then get very quiet or disappear completely after 5-10 seconds. It might take longer for some engines, but that's how mine acts. When I had a car with bad rob bearings, they would sounds worse under hard acceleration/heavy load. I don't think that should affect piston slap very much. How many miles are on the engine? If all else fails, it's not that bad of a job to drop the oil pan and check the rod bearingsd
has about 108K. Sat for 19 years, ran the engine quite a bit throughout the building process but never really drove it. Problems never occurred until I actually drove it a couple times
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Unread Yesterday, 09:52 PM   #6
kirbot
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Quote:
Originally Posted by Chstock View Post
has about 108K. Sat for 19 years, ran the engine quite a bit throughout the building process but never really drove it. Problems never occurred until I actually drove it a couple times
Is it the 258 I6 or the 4cyl?

My money is on piston slap. I'd be very surprised to see bad rod bearings at 108k

My 258 has about 100k and the rod bearings look almost new. It does have some piston slap though, but that's not really something to worry about.
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Unread Yesterday, 09:58 PM   #7
Chstock
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Quote:
Originally Posted by kirbot
Is it the 258 I6 or the 4cyl? My money is on piston slap. I'd be very surprised to see bad rod bearings at 108k My 258 has about 100k and the rod bearings look almost new. It does have some piston slap though, but that's not really something to worry about.
258! What's the fix for piston slap?
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Unread Yesterday, 10:05 PM   #8
kirbot
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Quote:
Originally Posted by Chstock View Post
258! What's the fix for piston slap?
Short of completely tearing down the engine for a rebuild.... Just leave it as is.
Almost every Jeep I6 out there does it. It should run for another 100k without causing any problems.
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Unread Yesterday, 10:44 PM   #9
BagusJeep
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Piston slap is from excessive clearance between the piston and the cylinder bore. You hear it at start up and it may disappear shortly as the piston warms up and expands. Any piston will have some symptoms of this, it is normal, but some you can hear and if you can hear it there must be a problem with clearances.

One way to check for piston slap:

1. Remove the spark plugs, leaving number one in place. I suggest this as it will make it darn easier to turn the engine by hand and you need to concentrate on each cylinder in turn.
2. Turn the engine over until you feel it becoming harder as number one piston comes up on the compression stroke.
3. Turn against compression until the piston is about half way up the cylinder.
4. You now want to rock the position so I suggest you grab hold of the fan or fan belt. Rock the crank back and forth.
5. You are listening for a metallic knocking sound.
6. If you hear a knock, you have piston slap. That is the sound of a piston going sideways in the bore and pinging off the side of the cylinder.
7. You can work through each cylinder in turn.
8. I have a mechanic's stethoscope, cost $10, that will allow you to hear that internal pinging a lot more clearly.

A rod knock is a rhythmic bang that increases in speed with the engine. The bearings are coated and oil starvation strips it off, so the steel surfaces eat each other rather than just the coating. The gap creates the bang and you will hear it at bottom and top of the stroke. However if you are rocking the crank you may also hear it. As stated dropping the pan and inspecting big end bearings is not a huge job and may save the engine. You can buy replacement bearing shells quite reasonably.
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Unread Today, 04:53 AM   #10
jeepdaddy2000
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Might get a second opinion.
Lifter noise from a stuck/collapsed lifter can be mistaken for bottom end issues. Lifter issues are common on engines that have been sitting for long periods of time.
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Unread Today, 04:55 AM   #11
Mike Romain
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Was the engine fully warmed up when the mechanic pointed out the knock? If so he was unfortunately likely correct.

How is the oil pressure? Usually it will be real low at idle if the crank bearings are getting sloppy.
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Unread Today, 07:05 AM   #12
Chstock
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Quote:
Originally Posted by Mike Romain
Was the engine fully warmed up when the mechanic pointed out the knock? If so he was unfortunately likely correct. How is the oil pressure? Usually it will be real low at idle if the crank bearings are getting sloppy.
I'll have to check on all that stuff when I go out there. My grandpa was actually the one who the mechanic showed it to. The jeep was originally in there because it wouldn't idle and died at stop signs
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