by bgsengine » Sat Sep 11, 2021 5:44 pm
Lot of the piston scuffing (which is what you are seeing, if it is just "fingernail catcher" deep) will be on the thrust faces, and remember, the scoring and scratching is typically BELOW the ring travel area - rings will re-cut the microscopic scratching left by bits of carbon that come loose, but there's no such shearing/polishing action on the skirt. However, if you look further down the bore below the piston ring travel you may see similar scuffing (but hard to get a fingernail in there in the right orientation to get a catch)
Note the thrust faces of piston will be slightly different - the combustion pressures forcing piston down, and the rod angle, are more severe which will slam the piston harder into the bore on the downstroke (and which is more susceptible to carbon buildup flakes, as it is just under the area between valves and bore - bore will be thinner as well, due to the valve guides on that edge, smaller cooling fins, greater heat, and expand differently) the tiny bits of carbon are what cause the scuffing (which as far as the bore goes are then cut away and re-polished as rings move over the area - not so with piston skirt) in fact on an engine well maintained you may notice discrepancy (the more hours engine has, the more carbon coming loose and down the combustion thrust face) between the thrust faces as maintenance generally resolves dirty/contaminated/carbonized oil, but less thought is given to de-carbonization of the combustion chamber...
Make more sense now? In other words, it is fairly normal for any engine with more than typical 3-4 year old homeowner pushmower hours. You'll see this also to varying degrees in cars, diesels, and in fact pretty much any internal combustion engine that has had a chance to build up any level of carbon deposits - it just isn't quite as noticeable in bigger water-cooled engines as thermal expansion properties are a bit different, and automotive type pistons (and performance engine pistons) may typically have a different piston coating, as well as cast iron cylinders (so piston skirts can be much harder alloy) as those pistons come out looking more "polished" on their thrust faces (but on a microscopic level, it'd look identical to your aluminum kool-bore pistons)
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)