OEM vs ARP Head Studs: What's the Difference?

Firstly, Studs vs. Bolts: Which is Better?

For most applications, head studs are the better, recommended choice for standard engine builds, and invariably the only choice for performance BMW builds. Using studs not only makes reassembly much easier, especially for performance engines where servicing must be frequent, but also ensures proper cylinder head and gasket alignment. 

When using bolts to secure the cylinder head, the fastener itself is twisted into proper torque reading, forcing the fastener to react to two forces simultaneously: the twist and the downward torque onto the head. Alternatively, studs are installed in a ‘relaxed mode.’ The stud is installed hand-tight, then torqued down so that the stud will only stretch on the vertical axis. As a result, studs provide consistent and accurate torque loading.

So, Why ARP Studs?

When reassembling your high performance BMW engine, one of the most important upgrades you can make is replacing your OEM head bolts with Automotive Racing Products (ARP) Head Studs. It's no secret that nearly every top engine builder across the globe relies on ARP head studs for their engine rebuilds, from modest power upgrades all the way up to full-race, ultra high performance power plants. So, why ARP more than any other brand? Simply, there is no better stud on the market.

1. Material: ARP only uses the highest quality alloy steels available for their hardware, including ARP2000, L19, and Custom Age 625+. These materials offer much greater tensile strength, fatigue resistance, and overall durability than the materials used in OEM bolts. More information on ARP metals is available here.
2. Heat Treatment: After careful material selection, each ARP Head Stud is carefully heat treated to ensure complete individual heat penetration, then centerless ground to ensure it is as close to perfectly concentric as possible. 
3. Fatigue Strength: Another practice that sets ARP apart from the rest is that ARP studs are thread rolled after heat-treat, which gives them about 2000% (20 times) better fatigue strength than studs threaded prior to heat treatment. Due to the extra wear on tooling, this process is more costly, but undeniably wields a better product.
4. Exacting Standards: There is no better proof of ARP’s stringent standards than during assembly. When gaskets and cylinder heads glide smoothly and easily into perfect alignment, an experience you will not get with lower quality, bargain head studs. ARP studs feature more precise fitment than any other fastener, ensuring the best possible torque and clamping force.

OEM Bolt Deformation and Stretching

A major fault of OEM bolts is that they are known to stretch and deform for a variety of reasons, including Torque-to-Yield Design. Most BMW OEM head bolts are designed as torque-to-yield bolts, meaning they are tightened beyond their elastic limit during initial installation and intended to be used only once. In other words, OEM bolts are designed to stretch during initial torquing to provide the necessary tension for proper clamping. These bolts absolutely cannot be reused as they are compromised structurally after initial installation and will be unable to maintain required tension, leading to insufficient and uneven clamping force across the cylinder head.

OEM Head bolts also weaken over time due to fatigue and wear, causing them to stretch under pressure. As the engine heats and cools during operation, the cylinder head will expand and retract. When the head bolts are no longer able to accommodate these expansions, they stretch.

Upgrading to high-quality ARP Head Studs and following recommended torque specs and installation procedure will ensure proper engine performance and longevity.

Conclusion

Overall, ARP Head Bolts offer a greater, more even clamping force, increased durability, and better resistance to wear than OEM head bolts. Constructed from higher quality materials and manufactured with much higher precision, standards, and consistency than OEM fasteners, ARP head studs offer predictably reliable performance and engine longevity, especially in high-performance applications where engine stress and cylinder pressure are greater. 

Always consult with a professional engine builder regarding the installation of fasteners. 

Thank you to our friends at Automotive Racing Products for sharing their expansive engineering knowledge with us!