ARP head studs for your Subaru WRX and STI ARP uses a premium grade 8740 alloy that is rated far superior to "aircraft" quality. Then, each stud is placed vertically in special racks and precisely heat-treated to 200,000 psi. These studs use a 12-point style nut and are OE-11mm sized. The head of the stud uses a 4mm Allen to thread the studs into the block.

ARP head studs are manufactured to such exacting tolerances, you will note that gaskets and cylinder heads literally glide into position and are perfectly aligned. ARP studs are thread rolled after heat-treat, which gives them about 1000% (that's ten times) better fatigue strength than those studs that are threaded prior to heat-treat (a very common industry practice). It costs a lot more to do it this way, because it's tough on tooling, but the results are well worth the extra effort. Premium quality parallel ground washers are also included with each kit. ARP head studs are a must for performance engines seeing increased boost, and they are truly the best on the market for your WRX or STI.