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It makes no sense to use the stock pistons with an aftermarket rod. The pistons fail right around the same HP level the rods do.....
Then there's the problem of fitting the small end of the rod into the stock piston--bad idea to machine them. You'll be hammering on the wrist pin bushing with that end mill during the process. BAD idea.
Last year we did a motor for someone and we tried to use the stock pistons only to discover the rods don't fit without a good deal of work. In that case we used CP off the shelf 10.5:1 that don't cost a whole heck of a lot. Driving it after the fact even NA? Not a huge difference in power. Maybe 10-15 hp or so, but with the ability to run crappier gas and much more boost. If you get the high boost pulley with any of the SC kits you'll more than make up for the slight loss of compression, but gain a whole lot more reliability. Stock pistons are hypereutectic and don't expand a whole lot, but they still expand and run tight end gaps. Get hot on track with them and it's bad news. A forged piston with 3.5-4 thousands clearance will do much better on track with a .017-.018 top ring gap, and has zero piston slap hot or cold on the FA20. Every motor we've put in the last few months has run just that, and you'll be hard pressed to tell it isn't stock when you fire it up.
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