Thread: Casue for 1 melted piston?

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70's something small block 350, bored .030 over, everything else is stock with quadrajet and HEI ignition and double hump heads.(this is in a boat btw) She wasnt running up to par so i threw a new set of spark plugs in her, she was running like a raped ape. after a little bit i started to hear some detonation so i backed out of it and didn't ever give it any more throttle. Once i brought it back down to idle there was an insane amount of oil smoke out of the exhaust. Few days later we take her out again not too much throttle, when she got up to temperature, the oil pressure dropped to zero, and the engine speed would decrease on its on. Well we get her home, tear the motor out and tear it down. Cylinder number 4 piston was melted, the bottom(outside) edge or the piston and rings were completely gone. I'm just trying to figure out what could have caused this so it doesn't happen again. normally i would say a timing or fuel issue, but with it being carbureted you would think that it would have affected all 8 cylinders? a buddy of mine was looking at the heads and noticed that cylinder 4 is where the egr port is so he thinks if the intake gasket had developed a leak it would casue just that one cylinder to run lean? can anyone confirm or deny this theory? let me hear what you guys have to say

Seems to me that you are describing exactly what happened to this engine. Note the piston damage at 12 and 1 o'clock of the piston. ( Yes that is the rings you are looking at ). It can effect 1 cylinder and not all 8.

det1.jpg

This damage on this one was purely detonation damage. It can be caused by Timing, too low an octane fuel, wrong spark plug temperature range. A vacume advance not operating correctly can do this also.

Yea thats what i figured was in ignition timing issue, I just assumed it would affect all 8 and not just one particular cylinder. As far as the vacuum advance not operating correctly, its a marine engine and as I have learned tearing this thing apart and doing my research, the vacuum advance on a marine engine never gets used, so i'm switching over to a marine all mechanical advance dizzy

Originally Posted by Titan uranus

I just assumed it would affect all 8 and not just one particular cylinder.

If you think about this for a minute, you'll realize that there is one best cylinder out of 8, one worst cylinder out of 8 and the rest of them fall somewhere in between. There are just too many variables to contend with. The block deck height can vary from one corner to the other and be totally different from the other side, crank stroke can be off by several thousandths, rod lengths vary widely, piston compression heights are off a little one to the other, combustion chambers aren't even close to being the same volume in most cases, some valve seats are closer to the block deck than others, some valves are longer or shorter than others, ignition timing varies widely due to play in the distributor drive gears and timing chain, dual-plane intake manifolds aren't even close to the same configuration left bank to right bank, etc., etc., etc.

You didn't have any rodents nesting in one of your intake runners did you?
HE! HE! I figured you covered the rest of the stuff Tech.
Kurt