[docrodg]

interesting, there are 2 possibilities off the top of my head. A carbon deposit, maybe from the engine overheating after it had excess buildup??? Bores are cast iron on those engines. Does it come off with a light scrape? That would be carbon deposits. If you had not said that the you stripped down the engine I would be tempted to say it was rust that had been treated with naval jelly. Of course, it could be that the engine had sat, rusted a bit in the bores, then been run or at least turned over and the rust was scraped off leaving behind the "Stain" of the affected metal which is still corroded just not red. I have seen that happen in cylinders before, the rings scrape off the rust but the sub-surface damage stays and looks like these pics. If rust then you need to hone the cylinders with stones and pray... if it goes deeper than you can hone then you will not be able to reuse the pistons as iron oxide is tougher than cast iron and will eat the rings up as well as the pistons.