I just checked your mods:
421 Cheetah Cub Play Port by HJR
Chariot Head 165psi
Shearer Inframe Exhaust
VForce Delta 2s
35mm PWKs w/ pod k&N & Boss Intake
Hotrods 4 mil long rod crank w/ Tz bearing
Hinson basket with straight cut gears
Lockout Cutch
Timing +7
Shift Star, Rod, & Drum mods
+4 Swingarm
What I believe happened is this:
+7 timing, combined with 165 PSI and ported, equals a need for race gas. AV gas might not be high enough octane for your setup. Jetting a little rich does not mean you don't have to worry about having enough octane to prevent detonation. I would back down to +4 timing when I put new pistons in if I were you. (Consider race gas, or bigger domes as well) Also remember, the temps just got a lot cooler, and the air is more dense, leading to a leaner condition for your bike. The load of pulling another quad is also a lot different than just you hauling ass alone. I think these factors lean to the detonation and the broke piston. Ive blown motors before, and I know what it takes to do so. Good Luck and keep us posted.