Piston skirt scoring

[creia]

If the cylinder bore and rings are in otherwise good condition (or have been rebored and replaced), is some scoring of the piston a big deal? Does the piston NEED to be replaced? Every now and then I take apart one of these 40-50 year old Briggs Flathead dinosaurs and see piston scoring, however, no scoring (or very little) on the cylinder wall. In the past I have always replaced scored pistons- just wondering if I really HAD to and what would happen if I reinstalled a scored piston.If the cylinder wall has no scoring. What is causing the piston to be scored?
Thank you,
Michael

[RoyM]

Minor scratching is normal and nothiing to worry about. Deep scratches (scoring) is due to abrasion from lack of mainenance which will also show on the cylinder wall especially on the Kool Bore engines you deal with.

[creia]

Thanks for the reply RoyM.
"Minor" can be very subjective term and will differ between individuals. That being said, I would not describe what I am seeing as "minor scratching". I can (very easily) catch a fingernail in all of these. In addition, they are concentrated on the thrust (front and back) sides of the piston which I would assume would take the most wear. I think what puzzels me most is why is there no scoring, or even moderate scratches, in the bore? If this was due to lack of maintenance (dirty oil and/or dirty air filter) wouldn't the bore show something as well?
Michael

[bgsengine]

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)

[creia]

Thank you Brian- Yes, I understand it better now.
Interesting what you said about the wear being different on the opposing thrust faces of the piston skirt. On closer inspection I can clearly see that the scoring/scuffing is more pronounced on the REAR thrust face nearest the valves. In addition, this engine I am currently working on (circa 1968) had a LOT of carbon buildup (more than I usually find) on top the piston, on the underside of the head, and around the valves. I never even attempt to run these old dinosaurs I take into inventory until I pull the head and clean off the carbon buildup so as to not have any more carbon deposits dislodge and cause any additional scratching/scuffing/scoring.
Michael

[creia]

Update:
After further inspection and measuring, the scoring/scratching/scuffing on the piston skirt is a moot point. The cylinder wall, although it "looked" good w/o any scoring, was worn way out of spec- BOTH out of round and tapered! This engine will be getting a O/S bore and new O/S piston/ring assembly.
Michael