Pro-x pistons are designed to be oem replacements, so similar design and materials. That would explain the differences in clearances the last poster noted.

 

They are starting to use high silicon aluminum that does noy expand as much so you can run it on the tight side

 

the auto pistons called hypereutectic are cast with higher silicon content than regular cast pistons. my street-strip ride uses trw forged pistons only because i was thinking about spraying it. two years later i picked up another 100 hp with better cam,intake ,carb selections and lots of tuning. 420 hp isnt bad for a 355 sbc. i cant spray now using the cast crank(breakage). those pro-x cast pistons are also quieter on startup. you can hear the difference . cold seizures are more of a problem with forged pistons also. its all in what you like. if people would learn how to jet properly ,they all would last longer! my r is only .030 on original jug. that dumonde is some pretty good stuff!! LONG LIVE THE 2 SMOKERS !!![img]i/expressions/face-icon-small-happy.gif[/img]

 

They are using silicon in forged pistons as well