Fox 2011+ GMC Sierra 2500 HD 1.5-2.5in Lift Rear 2.5 Performance Series Smooth Body IFP HTO ShockFOX 2. 5 Performance Series HTO shock is a bolt on shock loaded with position sensitive technology, vehicle specific valving, and larger pistons to improve on and off road performance. A type II anodized aluminum body efficiently dissipates heat and prevents corrosion. The 2. 5 inch shock body features an internal floating piston (IFP) to separate the shock oil from the high pressure nitrogen, optimizing damping. Hydraulic Top Out (HTO) technology