Upon setting overnight, I get a piston slap or light clicking upon startup which goes away upon warm-up. I am thinking the last new set of oil rings are broken in and piston skirting is worn down ...

Blow-by is often caused by worn or broken piston rings or a damaged piston itself. But modern turbo-diesels are also prone to problems with their crankcase ventilation and emissions-control systems ...

Piston rings look like simple pieces of metal. But they're the lifeblood of the ICE process, honed through decades of research.