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Paul K.

41 V12 Low Compression on One Bank

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Well, I replaced the burnt valve and #1 cylinder now has compression. The problem I have now is that the compression is now 40 psi lower on every cylinder on that bank. Prior to removing the cylinder head, ALL of the other good cylinders on the same bank had very close to 120 psi. The engine had copper head gaskets so I replaced it with same type as there were spares that came with the car. I have read here on the forum to use the newer FEL PRO type gasket, but I wanted to not risk uneven compression between banks using dissimilar gaskets. The replacement copper gasket looked like the same thickness as the one removed although it is hard to tell as they get mangled when taken apart. The only other item changed was that I cleaned the carbon from the combustion chambers. There was not an excessive build-up of carbon, certainly not enough to lower compression this much. Are there thicker and thinner or early and late style gaskets? Thanks in advance for your suggestions and ideas.

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I can't imagine there being enough difference in gasket thickness to account for *that* much compression loss. You must have an leak that is open to all the banks. Sorry I can't be of more help; some others here might have better ideas.

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did you recheck the other bank again? just to check your gauge? i sat and looked at my bare block last night and it does not make sense.

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Maybe a leak or restriction in your compression test set up? Sticking or faulty gauge? The only other thing that might cause this would be a timing gear that is out of sync but that would affect all the cylinders and you probably never touched that.

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Did you try shotting a little oil in a cylinder and recheck the compression? The cylinder walls may have washed out. Especially if you cleaned them while taking off the carbon. Just a thought George.

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I am baffled. This makes NO sense. Never touched the timing gear. My compression tester is good, and I checked the other "good" bank. All cylinders on the good bank are roughly 115-120 psi. I did a wet test on the low bank and the increase in compression was minimal maybe 5 psi. Going crazy here.

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Paul, If it was me, I would take that head off and try another head gasket. That's the only thing you changed and seems to me that the problem may be the gasket.

More than once I have over thought a problem.

Tom

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I agree. This is not rocket science. I do wonder if over time the copper gaskets flatten out and the heads need routine re-torquings? If that is the case, then over time the head will settle closer to the block and raise the compression. In other words, is this just because the gasket is new? Thanks!

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I think Tom is right. There must be something wrong with the head gasket you're using? I would use a new FelPro gasket and not worry about the very small change in compression ratio. If the block and cylinder head are clean and flat you should get a good seal without over torquing the head.

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And if you miss the one on E-Bay try Egge Machine in Santa Fe Springs. They list complete sets but may not offer just the head gaskets? Earle O. Brown is in PA but will give you quick service. Olson's Gasket here in Port Orchard (WA)offer quality gaskets for old cars and you don't have to buy a complete set. http://olsonsgaskets.com

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Check with Egge Machine in Santa Fe Springs - they have a lot of engine parts for our V-12's. If that fails, look up Best Gaskets - they are in SoCal too.

Norm

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You said that you removed carbon from the cylinders? If so, did you make sure that ALL of the carbon remnants were blown fron each cylinder. I have found that if all of the "junk" is not removed that it can get into the needle of your compression gauge and give false readings. Take a tire valve tool and remove the inlet needle from your gauge, clean it and try again. I find it hard to believe a "new" head gasket would give you bad readings on all cylinders, unless it got bent or had cracks in it from age. Also check to see that no carbon has become stuck under the valves, if you cleaned piston tops, carbon may have become stuck under the valves as you turned the engine over.

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Thanks Daddio, and everyone else that has racked their brains on this one. The compression gauge is fine as the compression checks good on the other bank. Its crazy. The good bank, which is the one I did not take apart is 115-120 PSI across and the bank with the new head gasket is about 75 lbs across. The low bank was just like the good one before taking apart with 115-120 across with the exception of the one dead hole with the burnt exhaust valve @ zero psi. The gasket is old likely 20 + years old, but it looked fine. I am just going to order a new gasket and start over.

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It wouldn't be something as simply as when you first checked it and got the 115lb reading you had the throttle wide open and when you re-tested and got the lower reading the throttle was closed. Just a thought.

David

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i had a similar problem with two cylinders and my problem turned out to be a warped aluminum head. i found another head and it cured problem. i used gaskets from earle brown that are made by felpro with no problem. david

post-42668-143138037231_thumb.jpg

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Well, I finally got around to getting the head back off to install the Fel Pro head gasket. I checked the head with a straight edge (machinist quality)and the largest space/uneveness of the head surface was at the MOST 1/64th of an inch and only in a couple of areas. To refresh everyone's memory here, this engine was running and had perfect compression of 115-120 in all cylinders with the exception of #1 that had 0 psi from a burnt valve. I replaced the valve and installed a new but old stock copper gasket that came with the car. I did not remove the other head. After the gasket change, there was compression in all of the cylinders, but it was low in all cylinders on that bank and at 75psi not the original 115-120. I then installed a new Fel Pro head gasket which has created no change from the previous copper style that originally gave me these crazy low compression results across the one bank. I am at the same place now, 75psi on the left bank with the new gasket, and 115-120 on the other side that was not taken apart. Uniformity is most important and I respect an earlier comment about the low compression readings possibly caused by cylinder wash-out during cleaning. A wet test only raised the compression about 5psi. What about ring expansion? On these old V12s, do the rings expand that much to cause this large variance between hot and cold? I realize the "good" side still has 115-120 psi when cold, but there was no cleaning done. Maybe the combination of cold and cleaning has caused this. For now, I will put everything back together and am going to try to get the engine to run, get it warm, cross my fingers and re-check the compression. Any ideas are appreciated, Thanks in advance.

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This might be a silly question but do the cylinder heads match? Do they both have the same "prefix" before the p/n under the front spark plug hole. Another possibility is that one of the heads may have been resufaced some time in the past?

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Thanks Peecher good thought but the head I am having the problem with was installed on the car and had good compression with the exception of the #1 cylinder with the burnt valve. The only things changed were the head gasket (twice with the same results) and the valve which brought the compression up to the level of the other cylinders on the same bank. Still going crazy.

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YOU REPLACED THE GASKET...WAS IT BLOWN?? WAS IT PASSING BETWEEN CYLINDERS?? ARE THE STUDS SCREWED ALL THE WAY IN..ARE U GETTING A GOOD TORQUE..DID U EVER START THE CAR ?? AND LET THE RINGS GET OILED BY RUNNING?? I WOULD HOPE BOTH HEADS HAVE BEEN MACHINED IN RECENT PAST ..IF NOT..MIGHT BE A GOOD IDEA..

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The head gasket was not blown. The head was removed to replace a burnt valve. The head has good torque and the engine ran well with the exception of a backfire from the burnt valve. As mentioned in an earlier post, Compression was 115-120 in 11 of the 12 cylinders prior to removing the head. The head looks good with a straight edge but regardless, it is not an alluminum head and sealed fine prior to removal. Now the bank with the new head gasket is 75 lbs. across all six holes. The side not taken apart is still at 115-120 lbs. As mentioned earlier, I am just going to start it, warm it up, and re-check the compression. I'll keep you posted.

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Paul

Check to make sure that you do not have any particles of carbon under the valves. I would cc the chambers of each cylinders to see if they are equal to each others. I would also start the engine give it a chance to warm up and than do another compression check.

Let us know

Frenchy

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OK, I guess I was over thinking this whole low compression thing but I got the engine started and it runs better than it did mostly due to the new valve and no backfire. I was able to warm the engine, check the compression in two cylinders and they have come up considerably, however the engine is experiencing slow cranking so I was unable to accurately check the others. I am constantly surprised by the learning curve I experience with these older engines. Compared to newer engines, I did not think there could be such a large drop in compression between hot and cold. Also, as thought of before, the cleaning of the cylinders and possible carbon bits causing the valves not to seal are all items that could have had a cumulative effect. My thanks to all suggestions. My next issue is my slow cranking problem... again.

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