Don't use old pistons. You can make plenty of power with low compression. Save your pennies until you can do a proper build.
I wouldn't necessarily use unknown old pistons but I will use inspected old pistons in good shape. Pennies aren't a problem I keep them in the trunks of my AAR, GTS, RTs and several other cars for traction. As much as my cheap nature and a desire to build some things like we use to have to at 16, anyone can go to the Summit website and order pretty much any part of an engine or a car for that matter, it's more fun in my view to build something with minimal investment and in many cases used parts. I have built more than I can count, some blue printed in excess and some thrown together at the track on borrowed parts and a wing and a prayer.
