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GARY F

Broken piece of piston

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37 Olds straight 8 flathead. I pulled a piston that had no compression. No rings broke. but a piece of piston between the top ring and second ring down was was broke. About an inch and a half long. Why would that break?  No score on cylinder wall.  I have three more to pull that had no compression.  I will pull them all to replace the rings  Thanks for your tips.

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You state low compression but the rings were intact, the only damage if I understand correctly is the ring land on the piston was broken on this piston. You also have 3 more with no-compression. If the compression was 0 psi on the cylinder that had the broken ring land I would check the valves and seats on that cylinder closely by putting a ring of light oil or diesel around the valves margin when they are fully closed and watch to see if it weeps through the valve and seat. Typically a piston that does not have any broken rings and no scoring will be low compression which still does not run correctly. If the other 3 cylinders pistons show the same damage I would suspect continous detonation in the cylinders from timing being advanced too far or someone was using ether to start/run the engine. if all 4 piston ring lands are broken I would replace all the pistons. If you reuse the pistons be sure to check ring side gap and end gap, both are equally important. Excessive ring side gap can also break the ring lands.

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Let me clear up myself. I said zero comp. These four cyl. had  had 25 to 30 lbs. comp.  I said zero  because they may as well been zero.  Thanks Ltc

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At 25-30 PSI there is almost certainly valve trouble. It is pretty hard to get down that low on rings alone. I imagine if it is that far apart you are probably grinding the valves anyway.

 

Broken lands, as Ltc4748 already mentioned, are usually due to detonation or starting fluid. A land could break from fatigue, but if you have more than one piston damaged, probably not ordinary fatigue.

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I agree that valves are likely a contributor to compression that low when there are no broken rings. I've seen badly scored cylinders and rings from over heated, seized and collapsed pistons and they still had most of their compression.

 

Paul

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Starting fluid is a quick way to ruin an engine.

If gas wont fire it there is something wrong.

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I just pulled the second piston with 25-30 comp. It also had broken land between the top ring and second ring down. No broken rings or scoring. All cyls. had no ridge on top.  As far as the valves go I previously freed  up the stuck lifters, lightly lapped the valves, all the adjustments were  good, 8 & 11 thou.  I also pulled the third piston with 25-30 comp. No broken rings or lands. but the top ring was frozen in grove. I dont know what the previous owner did but the last time it ran was 1975. Just for your update. Thanks

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You need new pistons and rings and a valve job.

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I would bet there is excessive taper in the bore, it tends to work the piston & rings too much causing failures.

Edited by JFranklin (see edit history)
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Broken ring lands, was common in the 350 diesel want to be starting fluid would break rings and the ring lands and would not show up as a,problem right away. 

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