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Ex-Military Spec 6JB compression issue


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  #21  
Old 02-10-2017, 12:55:53 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Ex-Military Spec 6JB compression issue

You have to set the valves at TDC Compression stroke. Could by accident, you set the valves at TDC between exhaust and intake? If so, clearances will not be correct. The fact that liquid is getting past the rings, means there is a mechanical problem. Bad or stuck rings in the ring groove, carbon on the rings, broken rings, worn out compression rings, bad piston. All mean the piston has to be pulled. It may be as simple as freeing and de-carboning the rings, to as complicated as replacing the rings and piston. I would also check that there is no carbon under the valve faces. If you have to replace rings, do both sides, as if one side is worn, then chances the other side will be worn too, even though it is now holding. You need all cylinders to have compression within 10% of value to each other, or you will have a work imbalance that will cause overheating in the good cylinder after the one with poor power, as the good cylinder tries to make up for the power loss. As an example, if the highest cylinder pressure has 100 PSI, the others need to be within 10 PSI. I would do a leakdown test to be sure you are not losing pressure thru the valves. You may have several issues here, both valve seating and ring issues. Personally, I would check both while you have the heads off.

There are several products that state they can free stuck rings, found at auto stores. One here in the US of A is Sea Foam Ring Free. Over on your side of the pond, there may be a comparable product. For myself, I don't take much stock in such products. Only a physical teardown can tell you exactly why you are losing compression or fluids past the rings. Let us know what you find.
Andrew
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  #22  
Old 02-10-2017, 03:22:37 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Ex-Military Spec 6JB compression issue

In reality, since he says one piston is holding the diesel fuel he put in both cylinders and the other cylinder is leaking out I am going to take a wild guess and say he has a cylinder sealing problem and not a leaking valve issue.
However, just to bolt the head back on without adjusting any valves and doing a leakdown test should take less than 30 minutes for peace of mind.
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  #23  
Old 02-10-2017, 05:52:03 PM
MadGutts MadGutts is offline
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Default Re: Ex-Military Spec 6JB compression issue

I hadn't touched the engine prior to it losing compression...
im the old skool - if it aint broke dont fix it!

But alas compression had gone from the 1st cylinder, and normal compression on 2nd... cranking by hand gives good resistance but second rotation nothing, ther should be compression on every rotation.

So today the piston came out, and it all looks fine, there is a slight difference in the gap between the open compression rings, 2mm at the most, and the oil seal looks ok too... no damage to bore or to piston...
The top compression ring is polished (shiney) but the second one is dirty in places... Could this be a tell tale?

other than that i cannot see anything bad on the piston, rings or bore!
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Old 02-10-2017, 06:49:47 PM
grif grif is offline
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Default Re: Ex-Military Spec 6JB compression issue

Valves are free?
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  #25  
Old 02-10-2017, 06:56:51 PM
MadGutts MadGutts is offline
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Default Re: Ex-Military Spec 6JB compression issue

yeah thoroughly checked top end before going to the bottom end! valves checked and reseated

and putting diesel in both cylinders to test for leaks, all diesel dissapeared from 1st cylinder, so there is a leak down the side of this piston
but i cannot see any faults
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  #26  
Old 02-10-2017, 06:58:34 PM
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Default Re: Ex-Military Spec 6JB compression issue

What is the ring endgap on each ring when they are inserted about 1 inch down the cylinder. 2mm is a huge endgap for rings , about .080, the manual calls for .010 to .020. What do the cylinder walls look like , do they still have a bit of crosshatch. Is there a ridge at the top of the cylinder? What is the taper on the cylinder top to bottom. What is the piston skirt clearance? Are the rings free to move in the ring lands not stuck in the compressed position. To me catastrophic compression failure means , burnt valve, misadjusted valve, broken rings, rings stuck in retracted position, broken ring lands, broke piston skirt, loose head bolts or blown head gasket. The fact that diesel leaks past the rings when it is sitting does not indicate to me that it has enough wear to completely lose compression, these are very low compression engines. I would bolt the head back on and do another compression test but squirt a bit of oil in the cyl to see if that brings up the compression substantially , or do a leakdown test as others have suggested to identify the source of the leakage. Show us some pics so we can see what you have been finding. Cheers Dan
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Old 02-10-2017, 07:13:18 PM
grif grif is offline
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Default Re: Ex-Military Spec 6JB compression issue

If it were me,,, I'd swap plugs and wires after making sure there was spark available for both cylinders.

We're missing something easy here. (I can hear my dad).
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Old 02-10-2017, 08:49:27 PM
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Default Re: Ex-Military Spec 6JB compression issue

Fisrt, If the valves were adjusted at the wrong TDC then they would be clattery as all get out running. We had people trying all the time to adjust their own valves on air cooled VWs and had more than one guy come in with the car sounding like it had BB's running through it. You would have to loosen the rocker nut a bit to get any clearance.
Second, if the 6jb is the same as a 7.5jb then it has a dual coil where both plugs fire at the same time, one cylinder getting a waste spark. So I would be inclined to test the spark by cranking the engine with the plug wires hooked to two spark plugs sitting external from the cylinder where you could see if you have spark at both plugs. This set might be a 6JB 50 cycle only running 1200 rpm?
Lastly, Big Block Chev has a good point to check ring gaps. A compression test would have been something I would have done first on both cylinders.
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Old 02-10-2017, 08:56:15 PM
Wayne 440 Wayne 440 is offline
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by grif View Post
...We're missing something easy here. (I can hear my dad).
Agree. I have been a victim of the "it can't possibly be..." line of thinking more than once.

So to me, the cause of the original problem is probably not that the rings won't hold liquid. I had a Ford 8N with so little compression that it would coast like the Wheel of Fortune when shut it off at idle, but it still pulled about as well as anyone else's. Was easy to start too, just put it in 2nd or 3rd, then reach over and give the back wheel a push.
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Old 02-10-2017, 08:57:07 PM
David C David C is offline
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Default Re: Ex-Military Spec 6JB compression issue

Really needed to do a compression check before the initial teardown. The next step would have been to put each cylinder at TDC on the compression stroke and apply air to the cylinders and see where the air was escaping.

I can't imagine what would fail in the piston and ring area while shutting down. I have a well neglected and used 2.5LK that had a .250" (yes 1/4") ring gap on the rings and it would still run. It had some blowby and smoked some, but it would start and run fairly well considering the ring gap.

David C.
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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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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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Old 02-11-2017, 09:02:15 AM
Wayne 440 Wayne 440 is offline
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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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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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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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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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Old 02-11-2017, 01:16:33 PM
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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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