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Ex-Military Spec 6JB compression issue


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  #31  
Old 02-10-2017, 09:02:02 PM
MadGutts MadGutts is offline
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Default Re: Ex-Military Spec 6JB compression issue

The valve clearances were reset to .12 intake, and .15. exhaust when i re assembled the engine.
I also have tried turning the engine over with NO rockers so the valves weren't opening.
Like i said there is good compression on one rotation of the crank, but the second rotation has nothing and you can hear the air hiss into the engine.

The endgap whilst in the cylinder i will have a look at tomorrow...
Cylinder walls are smooth - no scoring, and cant feel any cross hatch - someone else told me it could be glazed.
There is no ridge in the metal at the top of the cylinder, there was a carbon build up which i have cleaned prior to pulling out the piston.
Im not sure how to measure the taper...
Where would i measure the piston skirt clearance?
Rings freely move in the piston and i will try and get some pics tomorrow too
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  #32  
Old 02-11-2017, 12:30:19 AM
Chas183 Chas183 is offline
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Default Re: Ex-Military Spec 6JB compression issue

head gasket? have seen the gasket leak and make noise in the past.

chas183
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  #33  
Old 02-11-2017, 06:12:16 AM
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lokay5 lokay5 is offline
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Default Re: Ex-Military Spec 6JB compression issue

"The valve clearances were reset to .12 intake, and .15 exhaust"



There's your problem!
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  #34  
Old 02-11-2017, 09:02:15 AM
Wayne 440 Wayne 440 is online now
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by lokay5 View Post
"The valve clearances were reset to .12 intake, and .15 exhaust"



There's your problem!
Not likely- if the setting was otherwise properly done. That is about .005 in and .006 inch respectively. It is common in "metric" counties to express valve clearance in millimeters.

Or maybe so- the specification (after spec d) is .012 and .015 inch respectively.
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  #35  
Old 02-11-2017, 09:39:33 AM
MadGutts MadGutts is offline
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Default Re: Ex-Military Spec 6JB compression issue

not according to the Onan workshop manual 12 and 15 thou... and besides if there are NO ROCKERS connected then the valve spacing is irrelevant at this point!
no rockers = valve shut, still no compression on first cylinder and i can hear the air hissing down into the sump.

No im no expert on glazed cylinders and piston rings, only ever messed with small honda engines on lawnmowers etc, and the top end of classic minis...
here is the piston:


the top ring is polished and im guessing this was making a good seal, but the 2nd ring looks dull in places so could that be the fault, dont know what to look for in the oil ring though.

Heres another angle of the piston: and rings - no damage to piston or broken rings from what i can see


and here are the cylinders, not too easy to see, but the bottom in the picture has clear cross hatching, the top one has crosshatching in the lower part, but cant see or feel much in the top half...

So would getting a glazing stone run in these help?

---------- Post added at 01:39:33 PM ---------- Previous post was at 01:37:15 PM ----------

not sure where the cylinder pic went!
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  #36  
Old 02-11-2017, 10:49:48 AM
Dale Russell Dale Russell is offline
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Default Re: Ex-Military Spec 6JB compression issue

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.
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  #37  
Old 02-11-2017, 11:30:50 AM
MadGutts MadGutts is offline
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Default Re: Ex-Military Spec 6JB compression issue

no thats to show the different sizes between them
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  #38  
Old 02-11-2017, 12:41:43 PM
Jim Rankin Jim Rankin is offline
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Default Re: Ex-Military Spec 6JB compression issue

I believe I would get a hone and give that cylinder a quick (limited) dressing up, stagger those ring gaps and reinstall that piston as is. Of course a new set of rings would be the trick, but I am cheap.

Taper would be measured with a snap gauge and a good micrometer. If no ridge at the top of the cylinders and with cross hatching still visible, there is no reason to expect the cylinder is worn bad enough to cause problems. A quick check that the piston closely fits the cylinder (stick feeler gauge in between piston and cylinder if you want numbers) so you know it wasn't over bored/honed at some point in the past and ensuring that the rings have good spring to stick out past the piston lands would be about the extent of my investigation.

The issue of the rings having good spring is one I would consider carefully. if the one cylinder was overheating more than the other when it was shut down last time, the rings could have lost their temper.
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  #39  
Old 02-11-2017, 01:09:19 PM
beezerbill beezerbill is offline
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Default Re: Ex-Military Spec 6JB compression issue

How's the top of the piston look - any chance there is a hole in it? Or perhaps a crack? (Never seen a cracked piston crown.) Usually stuff like a holed piston is real obvious with burned metal sprayed around everywhere, and more common on two-strokes anyway. But something has to explain air gushing by the piston and into the crankcase.

Not a head gasket - no common pushrod tunnel for leaking air to find its way into the crankcase. And as you pointed out - not a valve sitting off its seat because no rockers (and wouldn't explain why you hear air rushing into the case anyway).

Badly warped rings? Never heard of this either but easy to check. Rings still lay flat? And if you put the rings in the cylinder by themselves and square them up and shine a light glancing off the cylinder wall do you see gaps or is the ring sitting against the wall all the way around? And why would a ring suddenly warp anyway?

Warped or mis-shapen cylinder? Again, why would it happen all at once?

Is the compression actually zero, or just low?

Very mysterious.
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  #40  
Old 02-11-2017, 01:16:33 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by beezerbill View Post
...Is the compression actually zero, or just low?

Very mysterious.
Yeah - Wish we had a compression number...
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