ARP Head Stud Kit, 05-10 Cobalt / HHRHave a 05 10 Cobalt HHR 2. 0L, 2. 2L, 2. 4L and want stronger head bolts? These Pro series studs from ARP are cold forged to ensure molecular integrity. They are heat treated prior to thread rolling and machining and are rated at 190,000 psi! The Pro series kits include 12 point nuts, and all kits come complete with hardened parallel washers for an even load distribution and accurate torque readings.