Chevy: Like a Knock

[Great68]

http://www.pistonslap.com

Looks like GM might be having a few problems with their newer motor lines.

 

[one2gamble]

actually its been a "problem" for a bit now but the media only likes to smack Ford around.

 

[gt/cs]

Okay, I've only just started reading about engine internals so I'm going to ask some questions. I hope someone will take pity on me and answer them

1) Piston slap is when the piston comes up out of the cylinder and smacks against the head? This makes a knocking sound?

2) In the first picture I'm supposed to be looking for signs of the piston hitting the edges of the combustion chambers as they curve up?

3) Second picture seems pretty clear to me, The piston is worn and malformed due to hitting the head, correct?

4) On the site there are pictures of the cylinder walls, the only one that looks really obviously wrong to me is #8, where there's a large gouge along one side of the wall. I assume this is from the connecting rod bending when the piston hit the head, causing the piston to push against the cylinder wall at an angle?

5) What's wrong with the #s 2, 4, and 6 cylinders? I can see some "ribbing" on the cylinder walls which I think is what I'm supposed to be seeing, but I can't think of what would cause it.

 

[rbohm]

actually piston slap is when the piston hits the cylinder wall with an audible noise. the piston does rock back and forth in the cylinder, but clearances are usually tight enough to prevent the noise being audible. as clearance grows, the slapping sounds becomes audible, usually when the engine is cold, and going away when the engine warms up and the clearances tighten up. if you look at the piston skirt, you will see an area that is scuffed. that is the kind of damage you get with piston slap. it causes excess wear, and higher friction in the cylinder, and requires more power to move the piston. at some point the engine will fail as a result of piston slap, but usuall the bearings get the blame for spinning rather than the piston. when the rod bearing clearances open up, if the head gasket, and the deck hieght are thin enough, then the added clearance can allow the piston to hit the head and cause real damage, like bent valve, broken pistons, bad head gasket, etc. the marks you see on the cylinder wall are scuff marks from the piston, and not actually gouges, though there will be some deformity of the cylinder wall.

 

[mustang70]

I remember when I had my T/A people were talking about problems with piston slap in the LT1's. I guess I got lucky because I never had that problem.

 

[Ozsum2]

Most slap goes away when the engine comes up to temp. Ford claims that it doesn't shorten engine life.

 

[Max Power]

Piston slap isn't necessarily a bad thing. I had a 3.0 Mitsy engine that had it from day one, it went away warm, and the motor ran for 185k before I sold it with no internal problems. In older engine, s;ap was common with forged pistons more than cast.

 

[gt/cs]

actually piston slap is when the piston hits the cylinder wall with an audible noise. the piston does rock back and forth in the cylinder, but clearances are usually tight enough to prevent the noise being audible. as clearance grows, the slapping sounds becomes audible, usually when the engine is cold, and going away when the engine warms up and the clearances tighten up. if you look at the piston skirt, you will see an area that is scuffed. that is the kind of damage you get with piston slap. it causes excess wear, and higher friction in the cylinder, and requires more power to move the piston. at some point the engine will fail as a result of piston slap, but usuall the bearings get the blame for spinning rather than the piston. when the rod bearing clearances open up, if the head gasket, and the deck hieght are thin enough, then the added clearance can allow the piston to hit the head and cause real damage, like bent valve, broken pistons, bad head gasket, etc. the marks you see on the cylinder wall are scuff marks from the piston, and not actually gouges, though there will be some deformity of the cylinder wall.

Thanks for the clarification

Is this one of those particuarly bad cases where the piston has actually come up out of the cylinder bore? I see the scuff marks on the skirt that you're talking about, but I also see that the top of the piston is very irregular. It's supposed to be smooth, right? Or is that just carbon buildup?

 

[bnickel]

that's most likely carbon buildup. another problem associated with the loose clearances that cause piston slap is blow-by, which is what the pic of the head is trying to show, lot's of oil buildup in the combustion chamber in the chamber on the far left. there might be some contact marks there as well but i can't be sure