Is this a sign of worn rings?

[240ZMan]

Take a look at this picture of the piston top from my L24. It's got 200K miles and is about to be replaced. The compression was low on all cylinders and rose with the addition of oil. Are those shiny clean areas around the edge of the piston from blowby due to worn rings?

I've got an L28 that is going in, and this block will likely not be used again by me, so I'm just asking as part of my ongoing Z education.

[MikeW]

I don't have an answer to your blowby question but I'd certainly be concerned about what looks like piston/valve interference, i.e., a timing chain slip sometime in the past.

[TomoHawk]

How much would the timing chain slip? Is that tensioner-thingy or the chain itself supposed to wear oout?

thx

[240ZMan]

What do you see that makes you think there was interference with the pistons and valves sometime in the past?

[MikeW]

Originally posted by 240ZMan

What do you see that makes you think there was interference with the pistons and valves sometime in the past?

I'm fairly certain that the pistons should have flat tops. There appear to be semi-circular impressions from both the intake and the exhaust valves.

I may be way off base here but I once had my timing chain tensioner break which caused the chain to slip. As I recall it bent some of the exhaust valves but I ended up replacing all of them to be safe. I admit that it was entirely my fault. I was replacing the head gasket and did the block of wood down the chain guide trick but apparently didn't wedge it in tight enough. The tensioner is spring loaded as well as controlled by oil pressure. It must have come out enough to cause too much tension on the chain. I therefore had to use a bit of force to get the timing chain sprocket back on and this extra force must have weakened the tensioner. It broke a few weeks later as I recall.

[240ZMan]

I understand your thinking. On this engine, although you can't see it in the picture, all the pistons have identical cutouts for the valves, so I don't think there was any interferance.

But anyone have any ideas on the discoloration (actually, lack of carbon) near the arrows in the picture?

[Zedrally]

I would think that no carbon indicates no combustion.

It may be that the air/fuel charge in the cylinder doesn't burn [explode] at the outer extremity of the combustion chamber.

Hence no carbon.

Have absolutely no idea if this is a answer but may be feasible?

[Ed]

I'm just guessing here but it looks almost like that area is discoloured because of heat. Like it gets too hot for carbon to build up. I don't know.

One way to check for blow-by would be to inspect the cylinder walls. There might be dark patches along the cylinder wall where the ring isn't making contact thus allowing blow-by.

[2ManyZs]

I think Ed may have just hit the nail on the head.

I'd say it was due to heat, probably due to increased friction with the cylinder walls due to worn rings. Or, it could possibly be due to pre-ignition caused by the carbon build up causing a hot spot on the piston crown not long ago.

Hard to tell with the pic, as it's hard to determine where exactly the block deck ends and the cylinder wall begins, it's also a slight possibility that it was due to contact with the headgasket that was intruding slightly into the combustion chamber, for any one of a few reasons that could cause that.

Blow-by would not show up conclusively on the piston crown, only on the cylinder wall as Ed said, or on the piston skirt at or below the rings lands.

[billcapp]

, probably due to increased friction with the cylinder walls due to worn rings.

I would think friction would be reduced with worn rings.

[hmsports]

Worn rings make the piston a bit looser in the cylinder but then the piston can wiggle around. I've had this happen on my of my engines and you can see very uneven wear -- indications of more friction.

[Ed]

Piston to bore size is very critical. Too much clearance and the piston can wiggle leaving scars on the cylinder wall, causing premature wear and can also effect compression. Not enough clearance and the piston can sieze in the cylinder. You must also remember that the piston will expand as will the cylinder but because one is made of aluminium and the other of iron you will have different expansion rates.

$.02