IPD is now offering cryo-treated C15 and C3500 head bolt kits that results in longer life with less internal stress and optimized tensile strength. Cryogenic heat treatment is a precisely engineered process of taking materials down to approximately -310 degrees F, keeping them at that temperature in liquid nitrogen to remove residual stresses, and then ramping temperature back to room temperature, followed by a mild heat treatment process. These bolts are made in the USA and cryo-treated in the USA. For more details you can download the sales sheets – C15 Cryo Bolts or C3500 Cryo Bolts.