old engine post-mortem

[rrubel]

Last week I had my mechanic drop the new engine into my Vette and give me back the old one (yes, it's *almost* ready to go; just need to finish a few things). Today I started looking for the broken piston that started this whole new-block issue.

Final result was that the bottom point of the #3 piston had cracked off (there's a piece that hangs lower than the rest of the piston, I assume to help keep it straight in the cylinder). The piston walls were quite worn, but worn unevenly, most likely due to that broken piece. Haven't finished looking at the others yet.

Not sure how to determine if a bearing is going bad. The ones for that piston had black streaks on them.

[RICHR]

 

[jmccloud]

Bad bearing

You could mic the journal for roundness and taper. Also use 'Plastigage' to check the bearing clearance. Usually a really bad bearing will have the babbit (the gray bearing surface) worn thru and copper will show.
Often a broken piston results from improper lube. Each rod bearing cap has a small slot in its upper mating surface with the rod. This slot spews oil onto the opposing piston. If the rods or caps are installed upside down, no lube, quickly worn rings, often cracked rings, worn piston skirts and engine death. (speaking from unfortunate experience) You might check the opposing rod cap (number 4) to see whether it was properly oilling number 3 piston.
Looking forward to hear your observations.

 

[rrubel]

Finally got around to taking apart the rest of the engine. The caps looked good, or at least were all consistent with each other. Found two pistons with broken rings, and one cylinder with VERY bad scoring (this held one of the broken ring'd pistons). Bearings all looked good except for the one on the broken piston (it was worn along one end). Three other pistons were worn more on one side than the other, the other four were even. And a couple more piston skirts were gouged or chipped, probably by the broken piece flying around, and there were a lot of scratches on the sides of some of the pistons.

Sounds like it was rapidly approaching the end of its life.

[RICHR]

 

[Ken]

... how to determine if a bearing is going bad. The ones for that piston had black streaks on them.

Sounds like a good indication there Rich. You'll be able to tell old, worn bearings, based on your experience with what new ones look like now.

 

[78SilvAnniv]

...a couple more piston skirts were gouged or chipped...

Can it be determined how the first piston skirt broke?
I am wondering if that first one broke, and these others that show gouges and chips, were interfering with any other moving parts in the crankcase, like the cam or counterweights on the crank?
It just seems odd that a skirt would break off without having connected with something solid enough to crack it. Is it even possible for something like that to happen with a stock engine?
(assuming the engine removed was original)
I ask because I recently read about 383 strokers and how clearances must be meticulously checked to prevent interference with moving parts.
Heidi

 

[rrubel]

I spun the crank a number of times, just watching things move (it's a neat sight!), and nothing looked close to hitting anything else.

When I bought the car it had a bad occasional high-RPM knock which I traced to a disconnected knock sensor (intentional, as the wire had been taped to the frame). I'm of the opinion that the piece cracked from knock, and eventually fell off and got smacked around the crank for a while before landing in the oil pan.

Would knock be a valid cause for broken rings, too? These both were the topmost rings.
[RICHR]