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Discussion Starter #1
dose any one know if performing a compression test results will vary on a la block weather or not the intake manifold is off or on?, the engine is out of the vehicle some what stripped down with the intake off
 

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I believe that should be OK as the service manual procedure wants the throttle wide open for a compression test.
The procedure also states that the spark plugs should all be removed and the engine warm. I don't know how important that is if you are only looking for a fairly even balance of compression between all 8 cylinders.

See post #10 in the following link:
http://moparforums.com/forums/f62/compression-test-11388/
 

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Always done with throttle open so no vacuum in the intake. All plugs should be out for maximum rotation speed and low load on the starter/battery. If the manifold is off, make extra sure that nothing can drop into the valves. Warm or cold should not matter that much (battery power drops quite a bit when cold). Evenness is the most important factor unless engine is just totally worn out.
 

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Discussion Starter #4
ok thanks, thats what I thought, looks like we got a problem here then, so next question, if there is two notches on the top of each pistin,(machained notches)is this a normal mark on a 360la or dose this mean they are already .020 over?
 

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Oversized pistons will indicate the overbore with the amount of overbore, as in .020 lightly stamped into the top of the piston.

The reason for the compression test with the engine warmed up is to allow the cylinders to be lubricated and any imperfections which could alter the sealing with a cold engine over a warm engine because it doesn't matter what the compression is when the engine is cold, it is what it is when the engine is warm. A really well assembled engine may not have much difference, but a worn engine could have greater than the magical ten percent difference between cylinders, so remember, we don't drive around with cold engines anyway, so, if greater than ten percent cold, warm the engine up and check again, and if it is the same, then you can know for sure there is a compression or sealing problem.
 

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Discussion Starter #6
well I cant do it warm as the engine is out and partially dissassembled but two cyls are drasticly low, yes they are beside each other, and no it isnt a head gasket, popped off the head just to be sure then inspected and lapped valves, then re-installed and re-tested with same results, did a wet test and both went up by 35lbs, tried a light hone while head was off also thats when I noticed the notchs, think we will be going .020 over here now,not a happy day!
 

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Did you mic or at least caliper the cylinders to determine if bore is greater than 4.00 or 4.020? That much difference does sound like rings. Any scratches in the cylinder walls? Sometimes rings that are broken don't show scratches.
 

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Discussion Starter #8
i didnt mic the cyls, and this motor was sitting a while, I think it was washed before being stored and a bit of water entered one cyl which layed at one side, not much but enough to rust the wall enough that when the hone when over it although it cleaned things up nicely, a small amount of a roughness was left on one area of the side of the wall, I think this was enough? although ironicly both adjacent clys are low enen though the one beside it looked good?
 

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Not unless it actually damaged the ring, and to do that it would have had to pit and break, and an oil check would show a difference. If it were that bad it wouldn't have been cleaned up with a light hone.
 

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Discussion Starter #10
well we might pop that piston out to inspect the rings but what I dont want to do is just patch it up, I would hate to re-ring just to find out the compression got only mildly better, sadly I think this is one for the machine shop
 

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Won't know until you pop the piston on the two cylinders out. If the head gasket had gotten a leak between the two cylinders and water entered the hot cylinders, the rings could have instantly laxed and the pressure drop just like that, and you would have no way of knowing for sure without the history of the engine. This wouldn't require any machining at all, but the rings on the pistons when the pistons were pulled would be tighter in the lan grooves and not expand, or expand softly when you pull them out of the cylinders but otherwise look fine.
 

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Discussion Starter #12
Ok,that's good info, not sure when I will get back to this as we are readying the 318 for install again as the original plan went, as this needs to be ready for car show season then its back to the 360
 
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