Why professionals insist using only Hi-Per Blue pistons: Hi-Per Blue pistons are reengineered original equipment design, manufactured with upgraded high performance blue AEM (ethylene acrylic elastomer) for superior thermal and chemical resistance, offering better fit and performance. Other pistons are often made of a less-tolerant alkyl acrylate copolymer (ACM). Why AEM is a better choice: AEM is an ASTM designation for Ethylene Acrylic Elastomer, a very high performance material used in applications where exceptional heat and oil resistance are required. Hi-Per Blue pistons offer superior compressive stress relaxation, dynamic fatigue resistance, vibration dampening, and resistance to acid condensates. Features and Benefits: Engineered utilizing best-in-class materials, meets or exceeds OEM specifications, better temperature range, higher tear strength, better bonding characteristics, designed for better seal support.