Our Horsepower Direct tech representatives are often asked what is the difference between a head stud and a head bolt. First of all, bolts are usually partially or fully threaded, while studs are typically threaded on both ends and often have an unthreaded middle section of the shaft. Studs do not have a head, whereas bolts always do. A engine head bolt tends to focus on stability and convenience. An engine head bolt is often thermally treated before they are machined or have thread cut into them. The head bolts provide high strength and durability and come packaged with washers or nuts.
Like the head bolts, head studs are also thermally treated at a high pressure to improve strength. Studs are shaped to make them concentric, requiring a series of cuts to produce a straight, balanced component. The head studs can be roll threaded before or after a heat treatment, but post-heating roll threads provide a higher mechanical strength and stress tolerant final product. Using head studs will make it easier to assemble an engine with the gasket and cylinder in proper alignment. Typically, head studs are more expensive than the equivalent head bolts.
A head bolt must be installed by torqueing it into place during an engine assembly. The bolt is rotated into its slot in order to engage the threads and secure it into place. This creates a twisting force and a vertical clamping force, meaning that the engine's combustion chamber begins accumulating load, the bolt with stretch and twist. Accordingly, the bolt is reacting to two different forces simultaneously, its capacity to secure the head is slightly reduced.
A head stud can be threaded into a slot up to "finger tightness," or the degree to which it would be tightened by hand. The cylinder head is installed and the nut it torqued into place against the stud. Because the the stud is torqued from a relaxed state, the pressure from the nut will make it stretch only along the vertical axis . Resulting in a more evenly distributed and more accurate torque load compared to that of the head bolt and reducing the chance of a head gasket failure. NEVER crank it in tightly using a jammed nut!
CONCLUSION
One of the main differences between head bolts and head studs involves the methods used to install or repair an engine. Higher-end head studs that have been specially designed within exact tolerances are capable of securely positioning the head gasket and the cylinders with near-perfect alignment. This makes it easier to assemble an engine using head studs. However, head bolts are far more convenient for disassembling an engine or for performing maintenance or part replacement. Many street-driven vehicles have master cylinders or other components that extend into the engine's compartment. In this case, head bolts allow the cylinders to be removed without removing the entire engine from the vehicle, as is necessary with head studs.
In conclusion, head studs are better suited for high performance vehicles with greater power requirements, while head bolts are more practical for your everyday, personal vehicle. Therefore, the fasteners are both basically the same, but just depends on the vehicle in question and the way in which it will be used.