Boosted engines create higher cylinder pressures which in turn require greater clamping force to hold the cylinder heads down. At some level of boost, the OE fasteners will fail and that will be a bad day. Studs versus bolts is a separate discussion for a later date. For now, we will stick to reviewing studs as they are far superior as a fastener type. We will use the LS engine as an example. 

Head stud upgrades to handle increasing boost can be summarized as “More”, “Better” & “Bigger” together with combinations thereof. More is going from an OE 4-bolt block to an aftermarket 6-bolt block. We will look at the Better and Bigger options in detail here.

Better Fasteners 

Head studs can be either Torque-to-Yield (TTY) or Torque-to-Spec. A stress-strain curve (Tensile Chart) shows that a given material first reversibly stretches in the elastic region. Once it reaches yield strength, stretching becomes irreversible. For that reason, TTY fasteners are not reusable. This allows the OEs to use less expensive materials as they literally push them past their limits. The chart below shows a generic Tensile Chart.

Aftermarket head studs use higher tensile strength steels and typically target torque at 80% of Yield. This is well within the Elastic region making these head studs reusable. Being well below Yield, they retain high tensile strength making them less prone to catastrophic failure compared to a TTY fastener.

Okay, so why do critical fasteners use the “80% of yield rule” instead of 90% or some other arbitrary amount? It’s because torque input is a notoriously unreliable method for controlling precise tightening as it measures friction rather than clamp load.  Tightening inconsistencies are a certainty. Decades of experience have shown that targeting 80% of yield builds in the appropriate safety margin to ensure the fastener will remain within its elastic limits. This avoids permanent deformation thereby preserving future predictability, performance and reusability.

Aftermarket head studs from P1 are typically from one of four classes of performance. E190 series parts are typically 190,000psi tensile strength, S220 is 220,000psi, H260 is 260,000psi, P270 is up to 270,000psi. Here’s a comparison of how those parts line up in practice in reference to common LS applications: 

 
While P1’s P270 series parts are made from Carpenter Custom Age 625 Plus (CA625+) and are offered for many other applications, the advantage of often being able to use a ½” nominal diameter fastener achieves a vastly improved application clamp load with more cost effective and readily available materials.  CA625+ is the top shelf fastener material but it’s always expensive and often difficult to source.

Going with a higher-grade steel is a great option when the engine is not coming out of  the vehicle for machining. The labor savings will more than cover the higher cost of better studs. Higher grade steel is also a great option for extreme builds that are already using either or both 6-bolt heads and ½” studs. 

Important Note: Do not buy off-brand or imported fasteners. They are not the same and not worth the risk. Also, only buy from trusted retailers to avoid getting counterfeits. 

Bigger Fasteners
Upgrading from 7/16” to 1/2” head studs really bumps up the clamping force. The increase is exponential rather than linear as clamping force is a function of cross-sectional area (radius squared). As shown in the previous table, the 1/2” studs provide roughly 40% more clamping force for both the E190 and S220 materials.  

As engine builders, machining for bigger head studs is our first upgrade choice. The engines is already apart so both block and heads will be on our CNCs anyway. Our customers appreciate that the larger head studs are a clearly visible upgrade.

Bigger & Better
The advantages of size and tensile strength are multiplicative. For example, clamping force of a ½” S220 head stud is 58% higher than an E190 7/16” head stud. 

Conclusions
1. You should have a strategy for improving clamping force for anything other than Light boost. 
2. The best choice(s) for Medium boost depends on machining access, budget, and boost level.
3. Heavy Boost warrants More, Better and Bigger head studs.
4. Do not trust your engine to fasteners of questionable origin, quality or authenticity. It is a very bad bet. 

Note: Special thanks to P1’s Shannon Strothers for both the data and editorial input.