Car: Toyota Corolla 2002 Engine: 1zzfe 235000mi

Hello, I removed my cylinder head to do a piston job and noticed some grey buildup on the outer wall of the combustion chambers (see picture). It’s the same on all 4 pistons, there’s is like a glaze / coating of the walls.

What could have caused this buildup?

I’m suspecting a bad head gasket, which I am going to replace anyways now that I’m here, but do I need to also resurface the head, or take care of something else?

Many thanks

enter image description here

  • Radiator sealant? Or perhaps one used the wrong coolant once?
    – Martin
    Commented Feb 14, 2021 at 17:51
  • 1
    More than likely it is the siica in the coolant that has separated over the years, nothing to worry about.
    – Moab
    Commented Feb 14, 2021 at 21:38

1 Answer 1


From your question, it appears you are completely rebuilding the engine. Part of the process of rebuilding an engine is to check the cylinder deck and head to ensure they are within flat tolerance. You do this by using a machined straight edge. If these areas are within tolerance, then you don't need to have then milled, just clean them up and Bob's your uncle.

The other thing you do is to have block completely cleaned, usually by hot tanking. This is a caustic solution which strips the block (if iron based) of all the years of crap which have built up on it, inside and out. You will also need the freeze (core) plugs replaced in the process. If the block is aluminum, then you can send it through a parts cleaner, which throws hot water with detergent at it. This doesn't work as well as a hot tank, but does a pretty good job and should clean the block pretty well.

I have no clue what the grey stuff is or what might be the cause. With 235k miles on the engine, there's a large chance for coolant contamination to occur. I do know it needs to be cleaned up before reassembly. It will interfere with cooling. Not that it will cause overheating, it's just with the layer of "stuff" on there, cooling will not happen as efficiently as it would without it being there.

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