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Unread 01-09-2010, 07:12 PM   #1
Sundowners
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1985 CJ7 
 
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Lower compression on one cyl after recent rebuild.

When I first got my jeep, I did a compression test and found that no.6 cylinder was lower than all the other cylinders. About 120 compaired to 140-150. I tore the jeep down to the frame, rebuilt the TC and tranny. I was going to use a 360 but found out that the block that I had was pretty beat up. I decided to stick with my 258. I started to tear down the 6 cyl expecting to see an typical motor that had 85,000 miles on it. To my surprize, the motor was just recently rebuilt with .040 over pistons, new water pump, roller timing chain etc. The cylinder walls still had some crosshatch pattern in them. Everything was very clean.

So why do you think that cylinder 6 had lower compression? The cyl wall looked good, there were two "shiny" lines in the cylinder wall but they are hardly noticable. there is still a cross hatch pattern.

What do you think I should do, redo the cross hatching and throw some new rings in or leave it the way it is?

Thanks,
Brian

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Unread 01-09-2010, 08:22 PM   #2
southshore30
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Yea, but measure your holes and piston to make sure its in spec and check the rings for proper gap. Maybe rebuilder just threw it in and didn't check the details. How do you know your low pressure in that hole is from the piston/cylinder? Could have been a valve not seating properly. Check the head, maybe have the valve's lapped while you have it off.
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Unread 01-10-2010, 08:06 AM   #3
JeepHammer
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1973 CJ5 
 
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Low compresson on one cylinder can be a number of things...

Rings that have larger gaps than recommended,
Ring gaps that line up and let the compression have a direct route to escape,
Piston slightly smaller than the others,
Larger radius on the piston edge or larger valve pockets than the others, (more common than you think)
One chamber cast with a larger combustion chamber than the others,
Valves that don't quite seal like they are supposed to,
Wiped out lobe on the camshaft,
Bent rocker arm or pushrod,
Valve spring that is giving up,
Worn valve guide,

Or something like a closer runner in the intake taking port velocity away from your low cylinder as it starts it's intake stroke... (very common)

Like I said, there are a NUMBER of things that will cause one cylinder to be a little lower than the rest,
And I've found out lately that #5 in AMC I-6 engines usually runs a little lower compression than the rest.
I don't know why, I've just observed that particular quirk in several engines over the past 5 years or so.
I don't have the machine shop anymore, so I haven't cut any of them up to find out what is causing it...
It just seems to be 'One of those things' I can't explain.
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Unread 01-11-2010, 05:07 AM   #4
Sundowners
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[QUOTE=JeepHammer;8660005]Low compresson on one cylinder can be a number of things...

Rings that have larger gaps than recommended,
Ring gaps that line up and let the compression have a direct route to escape,
Piston slightly smaller than the others,
Larger radius on the piston edge or larger valve pockets than the others, (more common than you think)
One chamber cast with a larger combustion chamber than the others,
Valves that don't quite seal like they are supposed to,
Wiped out lobe on the camshaft,
Bent rocker arm or pushrod,
Valve spring that is giving up,
Worn valve guide,

QUOTE]

Thank You!

I will print this out and use it as a guild to "what to look for".

Thanks,
Brian
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Unread 01-12-2010, 01:08 AM   #5
fredrok
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Dynamic tests are okay for a cursory check, but to troubleshoot and get an accurate assessment of cylinder health you really require a leak down test. There are sooooo many variables (as Hammer mentioned) that a 20# spread wouldn't be a reason enough alone for me to tear down. The leak down test checks how well the cylinder is sealing and nothing else. The readings aren't affected by cranking speed, cam timing, carbon deposits, etc. I know; after the fact....but next time.

BTW, a plus to this method is if you are losing cylinder pressure, you can find it by just listening. Of course, some blow by is normal past rings and even valves with enough time on them. Either test should be performed with the engine hot.
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Last edited by fredrok; 01-12-2010 at 01:25 AM..
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Unread 01-13-2010, 07:21 AM   #6
Sundowners
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I was going to do a leakdown test, my father has an actual leak down tester (Two gauge set up) from when he was an aircraft mechanic. The problem was that the engine has to be operating temp (i do believe) and my Engine is on the stand. Also, My father could not find his leak down tester, the last he used it was late 70's. Its probably stashed away somewhere.

So I have the Engine completely apart, This thing is like brand new, If I knew this before, I would have not touched it. I did notice one rod bearing that had a slight score it is, the relating crank surface is ok though. Also, one main bearing looks to have more wear on one end than the other, maybe the bearing wasn't seated all the way down? These maybe non-issues to someone with experience, I might be making more out of it than it is. Anyways, some of the cam bearings look to be marked up. I took great care taking the cam out so I don't think it was me, It might have happened when some one put the cam in? I'm thinking of replacing all the bearings for peace of mind.

How hard is it to replace cam bearings? Any special tips?

Also, Hammer metioned about ring grooves that are lined up. When I took out the pistons, the ring grooves where all over the place, what keeps the rings from rotating? I think that long time ago, I rebuilt an engine, It might have been a 2 stroke, that had a small pin in the ring groove to keep the rings from rotating, I'm going back 12+ years ago.

One more thing that I noticed; On the side if the connecting rods and connecting rod caps, there are numbers. I thought that these numbers referenced the location of the connecting rod in reference to the block, (1,2,3,4,5,6) Well I have 4,5,5,5,2,4. Maybe They had to replace the original rods with ones from other motors? The caps do correspond with the rods and are color coded with paint where the stampings are located.

blah blah blah, ok, I'll shut up for now,

Thanks for all your help!

Brian


Quote:
Originally Posted by fredrok View Post
Dynamic tests are okay for a cursory check, but to troubleshoot and get an accurate assessment of cylinder health you really require a leak down test. There are sooooo many variables (as Hammer mentioned) that a 20# spread wouldn't be a reason enough alone for me to tear down. The leak down test checks how well the cylinder is sealing and nothing else. The readings aren't affected by cranking speed, cam timing, carbon deposits, etc. I know; after the fact....but next time.

BTW, a plus to this method is if you are losing cylinder pressure, you can find it by just listening. Of course, some blow by is normal past rings and even valves with enough time on them. Either test should be performed with the engine hot.
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Unread 01-13-2010, 08:48 AM   #7
fredrok
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Quote:
Originally Posted by Sundowners View Post
I was going to do a leakdown test, my father has an actual leak down tester (Two gauge set up) from when he was an aircraft mechanic. The problem was that the engine has to be operating temp (i do believe) and my Engine is on the stand. Also, My father could not find his leak down tester, the last he used it was late 70's. Its probably stashed away somewhere.

>>And probably has rotted hoses anyway.

So I have the Engine completely apart, This thing is like brand new, If I knew this before, I would have not touched it. I did notice one rod bearing that had a slight score it is, the relating crank surface is ok though. Also, one main bearing looks to have more wear on one end than the other, maybe the bearing wasn't seated all the way down? These maybe non-issues to someone with experience, I might be making more out of it than it is.

>>Maybe, and they don't necessarily wear uniformally as some mains take more load than others. The score is not good though, so replace those bearings (and look for markings of oversizes).

Anyways, some of the cam bearings look to be marked up. I took great care taking the cam out so I don't think it was me, It might have happened when some one put the cam in? I'm thinking of replacing all the bearings for peace of mind.

>> They do get slightly marked up as it's almost impossible to keep the cam from making contact going in and out. (I use a looong bolt in the face for leverage)

How hard is it to replace cam bearings? Any special tips?

>> Piece of cake on the stand. Does require a special tool any machine shop will have. Maybe they'll rent it or you can bring in the block. Takes 15-20 minutes to remove and replace.

Also, Hammer metioned about ring grooves that are lined up. When I took out the pistons, the ring grooves where all over the place, what keeps the rings from rotating? I think that long time ago, I rebuilt an engine, It might have been a 2 stroke, that had a small pin in the ring groove to keep the rings from rotating, I'm going back 12+ years ago.

>> They rotate naturally. I have pulled many apart to find the gaps lined up. Nothing you can do about that.

One more thing that I noticed; On the side if the connecting rods and connecting rod caps, there are numbers. I thought that these numbers referenced the location of the connecting rod in reference to the block, (1,2,3,4,5,6) Well I have 4,5,5,5,2,4. Maybe They had to replace the original rods with ones from other motors? The caps do correspond with the rods and are color coded with paint where the stampings are located.

>> I have seen plenty with good, odd or no markings. I mark them appropriately on tear down if not already. Never mattered to me as I was always resizing them anyway, but your theory could be correct. Any cosmetic damage inside the block from scatter?

blah blah blah, ok, I'll shut up for now,

Thanks for all your help!

Good luck!

Brian

..........
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Unread 01-13-2010, 11:51 AM   #8
Cutlass327
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Some engine factories use a coding system for their bores vs pistons. They'll build the engine, measure the bore, and using the measurements decide which pistons to use. If the bore is x.xxx, they'd use an 'A' piston, if it is a x.xxy, it'd be a 'B' piston, and so on. Some also use numbers instead of letters. If those pistons were the originals, and it had never been bored, you'd probably be able to measure the '2' cyl and compare it to the '5' and see the difference in diameters. This was their way to compensate for machining tolerances - match piston to bore - piston machining tolerances gave them matches for bore tolerances.
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