DD15 liner? Need someone to offer a good explanation!

[schISM]

I would say it would be impossible if that’s a factory defect but that’s exactly what it looks like. The cutting process involves separate boring bars, a rougher and a finisher. It looks to me like the rougher insert could have broken leaving too much material for the finisher causing it to “chatter” when it took its turn. If it had happened anytime after assembly those marks would be running parallel to the travel of the piston not horizontal.