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 190,000 psi, ensuring complete heat penetration.Following heat-treat, each stud is centerless ground to make it as close to perfectly concentric as possible. This procedure involves about ten very slight cuts and results in an exceptionally straight part.

ARP 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.

ARP Subaru EJ20, EJ25, & EJ257 Main Bolts are crafted from 8740 forged chrome moly and feature generous under-head radius and rolled threads for excellent reliability. The threads are rolled after heat-treating, giving them 1000% longer fatigue life than ordinary bolts. Parallel-ground hardened washers are included. These are crankcase through bolts for all EJ series motors.