It *looks to me* as if you've not quite got through the hard skin on the left. Small iron castings tend to chill in spots where there's less bulk to hold the heat, and the iron is locally much harder at those points. Where it resists the tool you can get a glossed, rippled finish. Perfect cast iron has a bit of an open 'micro-Aero' texture with tiny graphite-lined surface porosities. In some deeper places you can get concentrations of graphite, and I'm thinking that may have happened to the lower lug shown in your second pic.

But really you need to waggle it about to get different lighting angles to get proper view of the surface.

Looking at the base of my 10V – made more than 20 years back – I don't think I machined it at all, but probably emeried it flat with wet-and-dry and WD40 on my bench drill stage. The flat faces of the bed and the standard, as well as the cylinder of course, are more critical, and you should find you've got enough material there to clear the chilled skin completely.

Edited By Mick B1 on 27/07/2019 21:42:40

Thinking about it, I had very similar on mine also. (Stuart 10V Kit from castings)

I actually didn't go all the way through to it was uniform, I just polished each of the surfaces. I didn't realise it was a difference in the metal structure.

Engine runs ok however, so

Del

My D10 project is the same way. Lots of chrome in the castings.