Piston Bore Stains

[1982FJoe40]

I'm wonder if anyone know what these stains are. I have them in a few of my piston bores. The markings are more like stains. They feel extremely smooth with no signs of gouging. I know my FJ had spent a lot of time parked before I bought it. I was thinking they could be from not be driven for a long time (oil stains?), but it's weird I don't see it in a my cylinders.

 

[BU5H P1G]

Might be glazed from lots of infrequent low idle operation??? just a thought.

 

[BU5H P1G]

I pulled this from a GM bulliten that shows engine failures due to aftermarket "power up" devices. The stains could be leopard spotting? It appears that this is normal, however if the bore has cross-hatching (i'm not sure) then this could be an issue if polished off.

Directly from the bulliten : Important: Cylinder Wall Spotting (commonly referred to leopard spots) is from the induction hardening process of the top 1/3 of the cylinder wall. This is normal for the Duramax ™ Diesel.

I'm sure someone with far greater knowledge than i will chime in soon

 

[shawnfj40]

Could be lots of things. If the 40 has set a lot over the years it is likely due to water/humidity/condensation. Then once run and heated sealed in with oil.

 

[1982FJoe40]

Thanks guys. It's always nice to get other perspectives. At least they don't feel like gouges in the cylinder wall.