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Old 07-14-2014, 06:46 AM
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compression failure

oh boy...I think I have painted myself into a corner again...have an older rebuilt engine that was never run and after installing it I find no "0" compression at all in cyl#2 of this 350sbc. All Others tested to 135psi except one leaky intake at 90 and this seemingly dead cylinder #2. I static lashed the valves twice and tried oil in the cylinder to find no change. So off comes the head only to find nothing apparently wrong there...head gasket good and took the head to a machine shop for test which it passed other than the leaky valve on #6... Valve action has been obsereved for both intake an exhaust...not measured but visually opening and closing as they should. Tried filling pistons with oil with head off and #2 cyl will leak down over a day or two while the adjacent cyl#4 has no perceptible loss. Pulled the engine and removed the piston...rings all look fine. Now what? Could these rings be bad...they were not stuck in any way to the ring lands but I did give it a treatment of atf too to try to loosen if it was stuck a week prior to removal.

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Old 07-14-2014, 07:55 AM
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Even engines with rings that are very worn or broken will usually have some compression. Zero compression should not be hard to find. My first guess is valve adjustment. I would start by backing off the rockers in that cylinder at tdc so they both have a little play in them. This will assure they are not to tight.
A long shot would be the intake runner is plugged and no fuel air mix is getting in to compress.
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Old 07-14-2014, 12:10 PM
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Quote:
Originally Posted by T-bucket23 View Post
A long shot would be the intake runner is plugged and no fuel air mix is getting in to compress.
I've seen this before, a shop rag stuffed into an intake runner and forgotten about.
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Old 07-14-2014, 01:27 PM
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Had a cylinder go completely dead on me like this once, and couldn't find the culprit. Finally found a valve spring that was broken very close to one end, then kinda nested down into itself. The valve was going up and down, but not quite closing.
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Old 07-14-2014, 07:55 PM
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hey guys..thanks for the response... And I agree that it should be relatively easy to pinpoint the problem here but i really did try everything...there are no obstructions...i checked and rechecked the lash and checked that cylinder at least 5 times at seprate occasions. with and without oil...once i filled the cylinder good it wold hold pressure till it started blowing by the ring gaps or "stuck rings" as i was led to beleive. I also Found that difference in the fluid leakdown so something is up with those rings even though I can't believe that would result in a zero compression reading. The engine is new old rebuild....probably ten years sitting..i had it two years waiting till the engine finally croaked in my el camino. I am no ring expert and maybe my leakdown test dosent' prove anything that isnt' normal but it was the only thing i could come up with. I had a competent machine shop check the head over and it is ok. I would assume they would have found a broken spring if it were present... maybe not. I will look more closely. I did do my lashing at tdc but had fully adjusted them to half travel as instructed in my chiltons. I fuly loosened each rocker on the second round relash too. All were adjusted to same condition so in my mind they'd all leak if any was going to. Got any feedback on the leakdown and how serious that might affect? It's looking more like I will need to add a new set of rings. I guess there's a first time for everything. Very slowly one at a time... slow learning curve.... beware craigs list.
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Old 07-14-2014, 08:28 PM
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Quote:
Originally Posted by spencely View Post
hey guys..thanks for the response... And I agree that it should be relatively easy to pinpoint the problem here but i really did try everything...there are no obstructions...i checked and rechecked the lash and checked that cylinder at least 5 times at seprate occasions. with and without oil...once i filled the cylinder good it wold hold pressure till it started blowing by the ring gaps or "stuck rings" as i was led to beleive. I also Found that difference in the fluid leakdown so something is up with those rings even though I can't believe that would result in a zero compression reading. The engine is new old rebuild....probably ten years sitting..i had it two years waiting till the engine finally croaked in my el camino. I am no ring expert and maybe my leakdown test dosent' prove anything that isnt' normal but it was the only thing i could come up with. I had a competent machine shop check the head over and it is ok. I would assume they would have found a broken spring if it were present... maybe not. I will look more closely. I did do my lashing at tdc but had fully adjusted them to half travel as instructed in my chiltons. I fuly loosened each rocker on the second round relash too. All were adjusted to same condition so in my mind they'd all leak if any was going to. Got any feedback on the leakdown and how serious that might affect? It's looking more like I will need to add a new set of rings. I guess there's a first time for everything. Very slowly one at a time... slow learning curve.... beware craigs list.
Bring the suspect cylinder up to tdc both valves closed, put some compressed air into the plug hole and see where is is coming out.
I dont understand "half travel. Could you elaborate on this.
normal adjustment is to make sure the cylinder is on tdc, back off lock nut. Start to tighten and stop when there is no more up and down play in the push rod. If the engine was not run after the rebuild the rings are probably not seated. With no compression at all I would think if you poured anything into the cylinder it would go right into teh pan in a few seconds. If the engine is still apart you could rtv a piece of plexiglass with a whole in it over that one cylinder to seal it and put some compresses air into it and see if it just blows right by. The only other thing I can think of, if it is leaking by the pistons that bad is that you have one piston that is the wrong size. A standard piston in a .060 over hole will leak pretty good.
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Old 07-14-2014, 09:03 PM
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Maybe you had a piece of carbon or other crap cought in a valve seat and it worked it's way out. Should have done the air leak down test before you toof anything apart.
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Old 07-15-2014, 01:58 AM
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. There are two top dead centers for every cylinder... the crank rotates two times for each one time the cam rotates... you prolly adjusted some of the valves on the wrong top dead center for #2 cylinder because the cam was holding the lifters up... rotate crank one turn and try again... don't try to feel for drag on the pushrod while turning it, move pushrod up and down and tighten nut until the click is no longer felt, then give a half turn more... lock the nut down unless it is self-locking... when done, rotate the crankshaft while watching the rocker arms and make sure all the rocker arms rock back and forth the same amount as engine turns two whole turns of the crankshaft...

. The pistons are in with rings... the machinist said the heads are good... that leaves the problem being the nut behind the steering wheel... LOL! ... (I assume you bolted all the rod caps on the rods and all 8 pistons are going up and down)...

. BTW, the rings should be installed with the ring end gaps of top and 2nd ring on opposite side of each piston...

. Compression test is done spinning the engine with the starter, all sparkplugs out, carb. WOT or removed... spin over about 5 times for each cylinder...

. Good luck...
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Old 07-15-2014, 02:46 AM
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Sounds like a collapsed ring, have you tried fitting the rings in the cylinder without the pistons?

Not that it matters, its out now...repair that cylinder with a ball hone and new rings...might as well do them all since your in there.

If everything is fairly new from a recent rebuild a quick ball hone and re-ring won't hurt anything from a clearances point of view.

Make sure you check the ring clearances in the ring lands and the pistons for proper fit in the cylinder, sounds like the previous rebuilder made a mistake or the engine was abused.

Consider this a fresh rebuild instead of trying to diagnose a problem which may end up being worse later once its reassembled.

Don't take anything for granted, check everything.
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Old 07-15-2014, 06:55 AM
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I have not removed the rings from the piston. they expand considerably from the installed position...excepting the oil control ring which seems to hold close to the stock bore size...I dont' know if that is correct but it is pretty obvious that that ring would have nothing to do with compression. I ran that engine for 3 -15 minute runs to break in cam s a minimum...it would not run under 1200 rpm...any lower and it would start missing and stall out. Put it in gear and the miss was quite pronounced. What exactly is a collapsed ring? there is no indication on cylinder wall of anything different from the other cylinders. Honing hatch is still visible in all cylinders. Yes, I think I am tempted to try to just replace the rings on that one piston as all the others were better than normal at a healthy 135psi.. 90 is an acceptable number in the info I have read. I would be a little worried about rehoning an assembled engine without being able to clean out the grit created by it. The suggestion is noted tho as I will probably have to buy a full set of rings? Everything is new from an unrecent rebuild. Pistons are new flat top 345nd std bore. I was told the engine was a low mileage core from the start. 96 block with center bolt 192heads. is there a free state measurement for the ring diameters? To see if maybe they have lost their tension tho I can't imagine them opening up much more than they are as they would fall off the piston at some point. I didn't think about the ring gap orientations till after I already screwed with it... I have doubts that that could make much difference, nothing prevents that from happening as a normal condition in a running engine, rings are free floating and will find their own balance. I may remove another piston just to compare.

Actual TDC is easily found with one finger on the plug hole. If you have pressure you are at TDC with both valves closed. I could actually hear the difference of the starter motor when checking that cylinder...no resistsance at all. you could hear a cyclic pattern in all the other cylinders checks. I did the compressed air check and that was the only way I could build pressure in the cylinder with 80 psi applied. I could hear air escaping but could not tell from where...my hearing sucks. I proceeded to test with reinstalling head and adding 1/2 qt of oil and gave it the pressure which it would hold for a few seconds till it started blowing by the rings. Had to be the rings as there is no oil in the head at all. Half travel is the designed position for the lifters to be adjusted to...one whole turn after tightening to the point of removing the freeplay. granted in this case I might have tried leaving them loose...too late now. Head was checked and ok'd by Hunts machine in Sch'dy...they are competent. And most definitely there is something wrong with the nut behind the wheel. Something serious I think.

I think that I shall re-ring at least that one piston... If not all. Like everybody else...I am trying to do it on the cheap or I wouldn't have bought an engine on craigs list in the first place. It's going into my 83 camino daily driver.... I thought some real power might be good for it if I can get it to go. As it is, I am already close to the cars value with my investment...dont' want a 6g car that cost me 10g to build. Next we'll be onto the built th350 I put behind it which may or may not have been a good investement.... if I can get the motor to run right.
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Old 07-15-2014, 08:38 AM
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A full turn is too much for lifter preload.
Since you have it apart have the piston checked to make sure the size matches the bore
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Old 07-15-2014, 09:05 AM
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hey t-bucket,,, my chiltons manual would disagree with you...one full turn is what they say...and all the others were adjusted to the same with no ill affect to them.

The piston is new and it is factory marked as I indicated which is the 350 std bore piston. I dont' think they make undersized pistons for the 350. I can measure it with my verniers ....as soon as I find out what size its supposed to be. Visually it is a pretty tight fit in the cylinder...maybe .015-.020 clear each side it seems.
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Old 07-15-2014, 09:29 AM
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Quote:
Originally Posted by spencely View Post
hey t-bucket,,, my chiltons manual would disagree with you...one full turn is what they say...and all the others were adjusted to the same with no ill affect to them.

The piston is new and it is factory marked as I indicated which is the 350 std bore piston. I dont' think they make undersized pistons for the 350. I can measure it with my verniers ....as soon as I find out what size its supposed to be. Visually it is a pretty tight fit in the cylinder...maybe .015-.020 clear each side it seems.
Very strange issue, I realize you adjusted them all the same way but 1/2 turn after 0 lash is pretty much the accepted standard.
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Old 07-15-2014, 10:57 AM
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Hmm..you know ...you may be right...i just googled valve lash proceedurs and they also said 1 turn is too much...wtf? Since when is chiltons givning out bogus info? I guess my 45 minutes of run time in 15 minute intervals was not enough to burn a valve if thats what was happening.
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Old 07-15-2014, 11:23 AM
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Is this a roller cam or flat tappet? Are the valves being opened on the dead cylinder or did a cam lobe get wiped off? Can you see the rings with the piston in its bore? Maybe the engine assembler forgot the rings on that cylinder? Lol
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