1453031 Piston As Fits Caterpillar ModelsOne New Aftermarket Replacement Track Adjuster Piston for Caterpillar (Cat) Excavators This aftermarket replacement track adjuster piston (also known as a recoil piston or Piston As) fits the undercarriage track adjuster assembly on Caterpillar (Cat) excavators. The piston works within the hydraulic track adjuster cylinder to regulate idler position and maintain correct track tension, absorbing impact loads from the undercarriage during operation.