Now thinking this might be spalling damage from cavitation in the water jacket, like what you get on the inside of tank armor from a non-penetrating round impact or explosion.
images.app.goo.gl/2MyRtRdW2BwW7prp6
Plenty of discussion of cylinder liner cavitation (mostly in the context of heavy duty diesels) out there, for example
http://www.constructionequipment.com...maintenanc...s
but I have not seen any mention of corresponding spalling inside the cylinder, so that’s just my speculation.
If that IS the cause, there will be corresponding pits inside the liner, and it might have been promoted by my practice of boiling the coolant to clear air pockets. Might have to figure out a vacuum fill method instead, a hassle I could do without.
Probably worth trying a different coolant (I used Shell last fill) and maybe changing the rad cap.
Can’t think of anything else.
There doesn't seem to be any mention of a procedure for changing the cylinder linere in the manuals I have, so maybe its considered a specialist machine shop job.