Originally Posted by Shaolin Crane
Not arguing, just stating my experiences. Probe extensively tests their pistons, something like 100k hours at red line. In some of the high winding motorcycle engine ive built I'd break a timing chain and the valve would leave its full impression on the piston. That bike is still going strong with the same owner who bought it from me 6 years ago. This was a 15:1 compression 36whp single cylinder thumper. Small impressions like that are nothing and have personally ran much worse for a similar amount of time. Gashed or grosh material removal anything where you can see that metal has been "moved" is the kind of stuff to look out for. Those are really no big deal. Its no difference then when a piston skirt has been shaved or valve reliefs have been cut or the dish has been opened up. I wouldnt worry about them even slightly. Hell the diesel guys take the 6.4 pistons and cut the dish open nearly 5cc then polish the edge and run 70-80psi on a hypereutectic.
Piston manufacturers typically have one as forged piston that can be machined into many different configurations. An area that has quite a bit of variability is the valve reliefs and so there is typically extra material in that area to accommodate all the different combinations. Having the valve hit in that area is one of the better ways to make contact with a piston. Unless you had huge reliefs because you had valves that barley fit in the head and a really high lift cam, you probably had some extra material. Also, that area is going to be predominantly in compression so that works in you favor too.
The OEMs are pretty well known to throw metal at pistons; it is cheap insurance. It doesn't surprise me the diesel guys are able to use their stock pistons in heavily boosted applications.
Machining aluminum is different than upsetting aluminum. Machining metal cuts and removes the material like when you opened up the valve reliefs or shaved a piston skirt. As long as a dull tool that generates a bunch of heat isn't used, everything is fine. Upsetting the material imparts strain into the part. That is when things get fuzzy. Different process conditions can produce vastly different results. For example, certain alloys will crack if they are upset at low temperatures versus high temperatures. Also, putting stain into a parts acts like an aging process. Aging is how you get the different -T8 or -T6 tempers after the same quench operation for the same alloy. The main reason that I get worried when I see damage is because I know of affects it MIGHT have but I don't have enough technical experience to be able to quantify if it happened or to what magnitude.
After all that hot air, the pistons are probably fine.