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Residue on newish piston

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I have put about 10 hours on an engine rebuild. On my way home tonight the engine was making some chattering noise, so I took the head off in search of a culprit. This is what I found. When I shined a flashlight on the piston, the residue appears white. When there is just ambient light, the residue appears brown. I can scrape it off with my fingernail. 

 

When I did the rebuild, I left the head alone despite it having 12k miles on it (financial reasons). 

 

What am I looking at here? I'm thinking it is oil getting in through a bad valve?

 

 

Edited by JustinSB

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I have put about 10 hours on an engine rebuild. On my way home tonight the engine was making some chattering noise, so I took the head off in search of a culprit. This is what I found. When I shined a flashlight on the piston, the residue appears white. When there is just ambient light, the residue appears brown. I can scrape it off with my fingernail. 

 

When I did the rebuild, I left the head alone despite it having 12k miles on it (financial reasons). 

 

What am I looking at here? I'm thinking it is oil getting in through a bad valve?

 

Well, it can only be combustion byproducts, oil or coolant.  Coolant usually leaves the piston really clean (think steam cleaned) and combustion byproducts in a properly running engine only leave a light layer of baked on carbon.

 

Since you didn't touch the cylinder head, my guess would be valve guides and/or valve stem seals... so long as the piston/cylinder/rings are good.

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might be too lean?  running a pipe and open airbox without re jetting?    what kind of fuel?

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91 octane pump gas. Running a big bore kit with a pipe and 3x3 mod. Moose jetting kit. I talked to Sisneros Speed Shop today and he seemed to think it is likely just junk from all the additives in gasoline these days.

 

Anyways I'm going to send the head out and make sure its done right.  

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