Wiseco or Athena are the only two manufacturers that I could find that still list a piston for the TRX350X.
Wiseco has 10.25:1 compression pistons in 81mm (std), .020 (81.5mm) and .040 (82.00mm) oversize for $150 for any one
Wiseco also has 12:1 high compression in the same sizes for $168 each
Athena has .040 oversize only for $189
These are piston kits with piston, rings, piston pin and piston pin cir-clips.
Wiseco (in the 10.25:1 compression) also offers a "kit" which in addition to the piston kit includes a gasket set for $190 - the gasket set is extra for the 12:1 compression piston at $39.50
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