Re: Hole in Piston, Expert Opinions Please !?!?!


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Posted by Alan Bowes on February 18, 1999 at 11:00:42:

In Reply to: Hole in Piston, Expert Opinions Please !?!?! posted by Sean Butler on February 17, 1999 at 22:22:19:

Sean, what does the hole look like? Especially the edges of the hole? Any sign of melting, or just a sharp, jagged edge? Sometimes damage from overheating won't look exactly like melting, but there would be signs of the metal being deformed without the usual stress cracks that would take place if the metal was bent to its breaking point at a lower temperature.

How did the spark plug look? The "hot plug" theory is actually a pretty good one, as a brightly glowing plug could contribute to preignition of the fuel-air mixture, and pinging on one cylinder is not always easy to notice, especially with all the gear/drivetrain noise.

As has been mentioned, an overly lean cylinder can run hotter, possibly causing damage. It could be due to a leak, but it would take a pretty good air leak to make that big of a difference, and I really don't think a loose spark plug could contribute enough air to make a significant difference in mixture. Besides, even if a very loose plug was allowing a bit of extra air to be sucked in on the intake stroke, it would also be allowing more air to leak out on the compression stroke, reducing combustion chamber pressures and lowering the temperature. However, you have to bear in mind that even on a normally functioning engine there will always be some cylinders that run measurably leaner than others, due primarily to the rather inefficient intake manifold and passage design, especially with a single, center carburetor. It could be that this happened to be the leanest-running cylinder and was the first to sustain damage.

Is there any sign of damage to the other pistons or valves?

Running the timing too far advanced can cause preignition. You can usually hear this as pinging, but as I mentioned above, if it happens to be taking place only on one cylinder, it's harder to detect. It will take place more easily on the leanest-running cylinder.

It could also be that this particular piston was just a poor-quality casting that finally gave out. If there is no sign of melting on the piston top or around the hole, this appears to be more likely.

By the way, if you're running the truck at higher elevations with a stock fuel system, it is probably running too rich, rather than too lean.

One other theory is that the piston may have been damaged at some earlier time, weakening a chunk of it.

Make sure you retrieve any bits of metal that might have ended up in the crankcase. Flush it well and try to account for all of the pieces. There is a chance that it could have done some damage to the oil pump and bearings if some small bits got past the oil pickup screen.

Hope that gives you a few leads.

Alan



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