It looks like paint(although not only on one cap and darker than factory).
And is the most plausible explanation as any kind of oil residue would not manifest itself in that area and not in green color.
What is plausible is the heads were rebuilt at some point and whoever did it was smart enough to indicate that after checking all caps nothing was machined.
And I still recommend against the wire brush method inside any engine parts.
No matter how much attention you pay,it is possible that something makes its way into the engine and creates damage.
And is the most plausible explanation as any kind of oil residue would not manifest itself in that area and not in green color.
What is plausible is the heads were rebuilt at some point and whoever did it was smart enough to indicate that after checking all caps nothing was machined.
And I still recommend against the wire brush method inside any engine parts.
No matter how much attention you pay,it is possible that something makes its way into the engine and creates damage.








by Gerry.





