Choosing the right head stud
A common misconception with fasteners is the more torque you can apply, the less likely it will be for joint separation to occur (lifting a cylinder head). Many companies continue to preach "bigger the better" due to the fact their fasteners can achieve higher torque than others which will allow you to have a safer engine. Though this is not untrue, it is also not a completely valid way to justify the fastener you are using. In today's blog we will dive into the mathematics behind clamp load and how it works.
Torque vs. Bolt Tension
One of the first aspects to understand with bolting a cylinder head down is that, as you'd expect, the bolt exerts a force on the head pushing it against the head gasket face of the block. To do this the bolt is tightened and put under tension, which we refer to as "bolt tension"
The relationship between torque and tension should be looked at carefully, the conditions experienced by a fastener can drastically change the outcome of your tightening (contamination, lubrication etc).
Torque is simply a measure of the twisting force required to spin the nut up along the threads of a bolt.
Tension is the stretch or elongation of a bolt that provides the clamping force of a joint.
Bolts are designed to stretch into their elastic range (this is where a material stretches with a force applied, but returns to its original state when the force is removed, much like stretching a spring a tiny bit), and this elongation is what gives force to clamp the joint together. Torque is an indirect indication of tension where friction is measured and correlated to a tension. Because torque is a measure of friction many factors can change the relationship and give you varying amounts of bolt tension for instance such as surface texture, rust, oil and debris. Torque/tension tables that have been developed, are all based on a formula similar to the ones below:
T = (K D P)/12
- T = Torque (ft-lbs)
- D = Nominal Diameter (inches)
- P = Desired Clamp Load Tension (lbs)
- K = Torque Coefficient (dimensionless)
The K value is dimensionless, it is simply a coefficient that is used as a multiplier to judge the various condition of the installed fastener. The value of K can range from 0.10 for a well lubricated, to over 0.30 for one that is contaminated and dirty. The values commonly used when calculating torque values are:
- 0.10 = Lubricated
- 0.20 = Dry
- 0.25 = Galvanized
For our purposes, a K value of 0.10 will be used as a constant due to most engine builders properly cleaning and lubricating their studs before installation. So now the real math begins!
Head Stud Selection
To begin we will look at several EJ Subaru studs on the market:
11mm (stock replacement) ARP2000 Max Clamp Load: 220,000 psi
11mm ARP L19 Max Clamp Load: 260,000 psi
11mm ARP 625+ Max Clamp Load: 280,000 psi
Outfront 1/2" (12.7mm) ARP2000 Max Clamp Load: 220,000 psi
P&L 13mm ARP2000 Max Clamp Load: 220,000 psi
IAG 14mm ARP2000 Max Clamp Load: 220,000 psi
The material strengths of these studs is listed, and is available on ARP's website. The published values on their website are considered "safe" values. They can be exceeded, and you *might* get away with it, but chances are you will harm your stud if you go beyond these values. For our study we will use the values ARP supplied, these values used are assumed as a hard limit, being that the bolt WILL yield at that point.
As some of you may notice, in our selection we do not even mention the OEM head bolts as an option. Due to the design there are a few differences that make them a poor comparison. First, the OEM bolts are torqued to yield, meaning they should not be reused nor can they be torqued higher than spec. Second, due to the nature of studs, you get better thread engagement, reduce friction as well as more distributed thread loading. You can see this in the picture to the right. The stud is able to bottom itself out which allows the lower threads to take some of the load. When tightening studs to a higher tension you are less likely to pull the threads out of your block.
For our analysis we used some very simple math, and a few assumptions. The first assumption is that all studs are properly lubricated and cleaned, giving a K=0.10 value as used in the torque equation. Second, the max clamp load was pulled off of ARP's website for each bolt. Third, we started at the advertised torque spec of each stud. This gave a good starting point to compare the bolts. Using the equation Torque= (K*D*P)/12, we can solve for P (clamp load), our final equation comes to P=(T*12)/(D*K) and we can solve the clamp load at advertised torque for each stud, shown below.
You can see each bolt acts pretty different from one another, both because of its size and because of the applied torque. The ARP2000 11mm out of the box stud, a proven solution for the general hobbiest, is at 77% of its capability, while the 14mm IAG stud is at a mere 54%. You can also look closely at the tension values, where the ARP 625+ is at 27,709 lb force @100ft-lbs, while the Outfront half inch is at 27,600 lb force @115 ft-lbs, and the P&L 13mm is at 25,791 lb force @110 ft-lbs. From this it can be concluded that at advertised specs the ARP 625+ is actually more efficient at clamping the cylinder head down.
By making the cross section of the bolt larger, the tension did not go up, in fact it went down. For any given torque, the smaller the stud, the larger the clamp load. The smaller studs are actually performing better at standard torques!! The down side to using a smaller stud is its capability. Even with the strongest ARP alloy, the 625+ reaches 100% capability at 41,500lbf, while the larger studs are able to achieve a larger clamp force in comparison before yielding.
Breaking down the info
With this info depending on your build we can properly size a head stud to suit your engine best. The ARP2000 is a great solution for the street car enthusiast who plans to see up to around 500whp. Torqued to ARP spec of 90 foot pounds there is minimal risk of having a head lift under boost. If you plan to go for more power or higher duty cycles repeatedly it would be wise to jump up to an L19 stud, or even the 625+. These upgraded 11mm studs will supply you with enough clamp load to make nearly any hp you want! When diving into the larger studs you need to evaluate your goals a little more thoroughly. For an Outfront 1/2", there would be no point in choosing their stud over the 625+ unless you are torquing at 135 ft-lbs or higher. At 135 ft-lbs you surpass the safe operating range of the 625+ (33,250lbf @120ftlbs tq) and start utilising the larger stud for its size. If you are looking at the P&L 13mm studs you want to torque at 140 ft-lbs or more to surpass the maximum safe clamp load of the ARP625+. Finally, if you're looking at purchasing the IAG 14mm studs you want to torque at 150 ft-lbs or higher.
The critical reason in pushing the larger studs to surpass the tension of their off the shelf counterparts has to do with bolt stretch (elongation). Bolt sizing and material is what determines the amount of stretch that occurs. Too little stretch and you can run into trouble if there is relaxation in the head or head gasket, this becomes an issue when you go to a larger stud that requires less stretch for the same bolt tension. This may not seem like a big deal but in a dynamic system with heat it can lead to catastrophic head gasket failure. For example if you imagine an 11mm stud stretched .02" to achieve a tension of 27,000lbs and a 14mm stud thretched .01" to achieve an equivalent 27,000lbs they will react very differently if their elongation is altered. If the component being clamped (head gasket) relaxes or shrinks (after a heat cycle) by .005" your 11mm stud will now be stretched only .015" and the 14mm will be stretched .005" which would result in the 11mm stud providing a clamp load of 27,000lbs*(.015"/.02")= 20,250lbs and the 14mm studs would provide 27,000lbs*(.005"/.01")= 13,500lbs. You can see that the 11mm stud was more resistant to this relaxation in the gasket because the stud was taken closer to its max tension. Relaxation in components is not usually a huge concern, however, in racecars with copper head gaskets this is a major aspect to consider and a torque check may be necesary after it has been run.
After reviewing all of the stud options for the EJ series engine, you can see that they are all extremely capable fasteners produced by successful companies. It is wise to keep in mind that when switching to a larger head stud it will require a larger amount of torque to bring the stud into its optimum operating range. When torquing studs as high as these are requiring, you risk pulling threads, distorting the block or head casing, having a failure due to lack of relaxed tension and distorting the cylinder bores after the head is installed. With this in mind it is wise to contact the supplying company to have the proper machining take place on your engine to mitigate these issues.