It is for good reason that virtually every top professional engine builder relies on ARP Pro Series head studs for their all out competition engines. Simply stated, there's not a better stud setup on the market today.
ARP uses a premium grade 8740 alloy that is rated far superior to "aircraft" quality. Each stud is placed vertically in special racks and precisely heat treated to 200,000 psi. This procedure ensures complete and uniform heat penetration, giving far superior results to the more usual practice of placing them all in a basket and hoping for the best.
Following the heat treatment, each stud is centreless ground to make it as close to concentric as possible. This procedure involves about ten very slight cuts, and results in an exceptionally straight part to start from. They are then Thread Rolled which, due to being done AFTER the heat treatment, gives them around 2000% (20 times) better fatigue strength than studs that are threaded prior to heat treatment. It costs a lot more to do this way, because it is very tough on tooling, but the results are well worth the extra effort.
These ARP head studs have been specifically developed for the Vauxhall Z20 engines.
Full set including a sachet of ARP's assembly lube, for correct torquing.