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Ex-Military Spec 6JB compression issue


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  #41  
Old 02-11-2017, 01:46:48 PM
Power Power is offline
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Default Re: Ex-Military Spec 6JB compression issue

Good close up pictures of ring lands all around, top and underside of piston would answer some questions.
What you say about the second compression ring is odd, but even if it was not there, the top one would provide compression.

You should follow previous comment about putting rings in bore and squaring them. I would put all 3 rings in bore, and check ring tension, look for space between ring and wall. If you do not have tools to measure taper, square compression ring at top of bore and slide it down checking ring gap at intervals. The amount of change in ring gap tells you how much cylinder taper you have. Shine a light in cylinder when you check gap. In unlikely event you see light between cylinder and ring in places, then you have a warped bore. From my experience, that is most unlikely.

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Old 02-11-2017, 01:53:50 PM
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Default Re: Ex-Military Spec 6JB compression issue

The second ring does look a bit suspect and the top ring is chrome so it would look shiny even if it wasn't in full contact. You have to remove the rings from the piston and insert them into the cylinder individually by themselves . Push them down using the top of the piston till they are about 1 inch down in the cylinder then use a feeler guage to see what the endgap is in the installed position. It is possible that the rings have lost their tension due to overheating but I wouldnt expect that to come on suddenly. What I find puzzling is the uneven wear on the second ring ,some spots it is shiny some it is not. . I am leaning towards a quick deglaze hone and trying to see if that seats the rings. I would use a ball hone to deglaze. It does look like someone has had the engine apart before as the blue stuff is not original I think. Would also be good to verify that he rings have been installed right side up in that case. I have been bit by that before. Cheers Dan
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Old 02-11-2017, 02:02:51 PM
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Default Re: Ex-Military Spec 6JB compression issue

Question to MadGutts and others here, The second ring looks suspicious to me as to its contact wear. I know from some engines the rings are directional. Is there marks on the rings for direction of installation? Could the second ring be upside down? If there are directional marks, they are usually right next to the gap.

Dan ^ was writing at the same time I was. He hit the reply button before I did. Looks like he and I are on the same wave length. That SECOND ring is not right.
Look at the first drawn diagram on this: http://www.hotrod.com/articles/hrdp-...ton-ring-tech/
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  #44  
Old 02-11-2017, 03:15:42 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Ex-Military Spec 6JB compression issue

To avoid problems, install new rings and de-glaze the cylinder walls with a ball hone. There is a definite problem with that second ring -either it has lost tension, or it was installed wrong. Re-installing it will not correct the contact problem. The ring is done. Install and hone on both sides. Be sure to follow installation directions. Rings are both directional AND ring groove specific - top ring in top groove ETC. Check the cylinders for out of round and bore taper. Piston fit must be correct. putting a standard piston in a .010 over bore will not work for long!

There are 2 ways to check piston to bore clearance. 1) install the piston with a brass feeler gage, try and slip it past the piston skirt. 2) using an outside micrometer, measure the OD of the piston skirt in several places. Then, using an inside micrometer, check the bore at the top, center and bottom. Subtract ID from OD and you have clearance value. Look to engine specs for indicated values.

Check the ring faces. Are they square faced with the bore or are they rounded? If rounded, the piston may be rocking excessively in the bore - too much clearance. Check ring end gap clearances. Excess gap will lose compression. Standard rings in an .010 over bore will not seal well. Are the wrist pin to piston and rod clearances correct? Rod bearings in good shape and of correct clearances?
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Old 02-11-2017, 04:11:29 PM
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Default Re: Ex-Military Spec 6JB compression issue

The compression is low... im going to see about de glazing the cylcinder and reassembling... will get a pressure reading once its back together

possibly a silly question...
Should the rings be sharp on the edge? the 2nd one down is a perfect square edge, but the top one has a champfer so its not a sharp edge?
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Old 02-11-2017, 04:32:35 PM
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Default Re: Ex-Military Spec 6JB compression issue

If the rings are properly seated they will be sharp. The top chrome ring has a chrome insert in the face of it you can see it if you look carefully but it too will be sharp. The other rings are slightly tapered top to bottom which is why they can only be installed in one direction. What is the history of the machine has it been apart before to your knowledge. Cheers Dan
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Old 02-11-2017, 07:34:55 PM
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by MadGutts View Post
The compression is low... im going to see about de glazing the cylcinder and reassembling... will get a pressure reading once its back together

possibly a silly question...
Should the rings be sharp on the edge? the 2nd one down is a perfect square edge, but the top one has a champfer so its not a sharp edge?
PLEASE, Before you jump ahead and reassemble it again, check the second ring for direction of install. That ring is not right from a wear standpoint. If you push the ring against the bottom of the ring grove OPPOSITE of the GAP, You should see a mark of some kind or TOP or UP that signifies the direction of the ring. ANY mark indicates UP toward the top of the piston.
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Old 02-11-2017, 07:59:28 PM
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Default Re: Ex-Military Spec 6JB compression issue

I have no idea if the engine has been apart before, but knowing the UK royal engineers, it will have been at some point!

The 2nd ring does have "TOP" stamped in the top side of it... i did wonder if that was supposed to be the top ring! but the top ring has nothing marked but there is a small dot punched into the top of the ring...
BU the top ring is definitely rounded off!
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Old 02-11-2017, 09:47:01 PM
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Default Re: Ex-Military Spec 6JB compression issue

Since you have both pistons out, how do the look of the rings compare to the rings in the cylinder that was not leaking by?

Can you compare how the rings fit in the grooves comparing the two? Maybe the ring grooves in the piston are worn out.

After looking at the picture I do not think you took out both pistons. I do think the top compression ring is worn out because of no sharp edges. It could have worn out the piston ring groove.
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Old 02-11-2017, 10:15:24 PM
John Hartman John Hartman is offline
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Default Re: Ex-Military Spec 6JB compression issue

I know nothing about your generator or engine. Reading all this is painful.

You cannot test compression of an engine by manually turning the crankshaft and listening for leakage. You cannot test compression or verify the condition of piston fit or ring condition by pouring fuel on top of the piston.

Put everything back together to the best of your ability, and look for an ignition problem.
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Old 02-11-2017, 10:20:33 PM
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Default Re: Ex-Military Spec 6JB compression issue

^^ Yeah, that's kinda what I was thinking back in Post #12...
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Old 02-11-2017, 10:44:49 PM
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Default Re: Ex-Military Spec 6JB compression issue

Jim and John-

Exactly. Engines do not instantly lose compression unless there is a catastrophic mechanical failure.
Concur- Obviously an ignition problem.
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Old 02-12-2017, 01:34:02 AM
John Hartman John Hartman is offline
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Default Re: Ex-Military Spec 6JB compression issue

Again, I know nothing about this particular unit, and little about generators.

I have seen very peculiar things happen with engines on dynamometers and connected to hydraulic pumps. I have seen Detroits, Macks, and Waukesha diesels run backwards.

If the engine is started under load it cannot overcome the applied resistance, but it will chug along as if the engine has a problem.

Engines need to be brought to rated speed before applying load.

There may be nothing wrong with this engine.
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Old 02-12-2017, 11:57:14 AM
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by John Hartman View Post
I know nothing about your generator or engine. Reading all this is painful.

Put everything back together to the best of your ability, and look for an ignition problem.
I agree completely To loose all compression you would have to have both top AND second ring broke or a hole in the piston
With no wear showing on the wall it is NOT worn out

I have seen engines run very well with 300,000 miles and .030 wear and rings with almost no tension left

Butch
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Old 02-12-2017, 02:07:12 PM
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Default Re: Ex-Military Spec 6JB compression issue

there is noting wrong with the ignition circuit and that has nothing to do with loss of compression!!!!
both spark plugs fire.

I haven't taken the other piston out to compare but i think that is the next point of call to see if there are any differences between them and the rings... i have also made a flat plate which i will bolt to the engine case and seal so i can get a compression reading from both pistons.
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Old 02-12-2017, 04:05:59 PM
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Default Re: Ex-Military Spec 6JB compression issue

You won't probably won't get a very good compression reading with flat plat bolted and sealed to the block, since there will be no way to get air in to compress. You can to a leak down test accurately with the sealed plate. I have over 40+ years of pulling on wrenches and I am pretty sure you don't have a ring problem. I believe there are several others folks with like wrench twisting experience that probably feel the same way I do.

David C.
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Old 02-12-2017, 05:39:55 PM
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Default Re: Ex-Military Spec 6JB compression issue

You can usually reuse the head gasket , at least with a bit of sealing goop , long enough to get a compression reading by bolting the head back on. It would be good to check the other cylinder rings as well. If the rings did not seat originally they usually never will without a glaze busting hone job. Cheers Dan
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Old 02-12-2017, 09:20:00 PM
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Default Re: Ex-Military Spec 6JB compression issue

[/COLOR]
Quote:
Originally Posted by David C View Post
You won't probably won't get a very good compression reading with flat plat bolted and sealed to the block, since there will be no way to get air in to compress. You can to a leak down test accurately with the sealed plate. I have over 40+ years of pulling on wrenches and I am pretty sure you don't have a ring problem. I believe there are several others folks with like wrench twisting experience that probably feel the same way I do.

David C.
I'm one of 'em David.

"The valve clearances were reset to .12 intake, and .15. exhaust when i re assembled the engine."

"...not according to the Onan workshop manual 12 and 15 thou"

.12 = one hundred twenty thousandths
.15 = one hundred fifty thousandths

Quote:
Originally Posted by Dale Russell View Post
In the pictures you show the piston's with the ring gap's all lined up together. I hope that isn't the way you install it in the Cylinder's. The ring gap's should be staggered 180 degrees around from each other. Just a thought!
When you are having a problem such as you have, you can't overlook anything because what you overlook may be where the problem is. My $.02 worth.
Irrelevant. Piston rings rotate when the engine is running.
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Old 02-13-2017, 09:43:55 AM
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Default Re: Ex-Military Spec 6JB compression issue

it was in thou - i don't have metric feeler gauges!

Shouldnt both piston rings have a sharp edge though as the top one is rounded?

for got to say the flat plate has a hole in for the compression gauge to sit in so when the piston is at its furthest down i will then hold in the gauge to get a reading - this way i can rule out the top of the engine
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Old 02-13-2017, 11:54:27 AM
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Default Re: Ex-Military Spec 6JB compression issue

Buy borrow rent or steal a leakdown tester and use it with your plate and compare both cylinders
Even on a good engine you need 4-5 revolutions WITH the valves operating to get proper reading

Butch
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