There is absolutely nothing wrong with using the OEM cast piston on the KX500. Like Polar-Bus, I have done a ton of top ends using the OEM piston without a single problem. The true test is that I raced on the same OEM piston for several seasons on one of my 250s. Sure, a forged piston is less likey to cause as much damage upon failiure, but if the cast piston is prone to breaking in your engine, you're likely to have other issues.
When Team Green desert raced the KX500, the stock OEM piston was used quite often. I realize that most people on this site are always going to try and seek more power, but there is a reason why so many pros who raced the KX500 kept the engine basically stock. You got to give the engineers credit who designed the motor (maybe I'm impartial because I'm an engineer!), because one of their major goals was realiabilty. Most heavily modified KX500 motors will never take the ruthless abuse of desert racing because they will either overheat, or simply not last that long. Yet, tons of "backyard" mechanics will insist that their mods aren't affecting realiability and only adding power. Their bikes will never be put to the test to prove this.
