Choosing whether to use head bolts or head studs in an automobile engine can be an important decision that affects the way a car runs. While they are both fasteners and ostensibly serve a similar function, there are a handful of differences that make studs and bolts distinct from one another. For one, 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. In addition, studs do not have a head, whereas bolts always do.
When evaluating the various benefits of studs and bolts, it is helpful to keep a number of considerations in mind. Ease of engine assembly and disassembly can be a significant factor, as well as torque pressure, gasket alignment, and overall engine performance. The power and acceleration potential of an engine often dictates the type of head fastener that will be used. For example, a high-end or racing model car will have drastically different engine fastener requirements than a vehicle designed for everyday purposes.
Head Fastener Design
Comparatively speaking, head bolt design tends to focus on stability and convenience. Engine head bolts can be cold-fabricated to increase their integrity, and are often thermally treated before they are machined or have threads cut into them. They provide relatively high strength and durability, and frequently come packaged with hardened washers or nuts. Some manufacturers produce head bolts with wide flange dimensions, which reduce the need for removing the bolts or the valvetrain when conducting cylinder maintenance.
Like bolts, head studs are also thermally treated at high pressure to improve structural strength. Studs are carefully shaped to make them concentric, usually requiring a series of cuts to produce a straight, balanced component. They can be roll threaded before or after heat treatment, although post-heating roll threads provide higher mechanical strength and stress tolerance in the final product. Some head studs are designed with gasket and cylinder alignment in mind, and they are typically more expensive than the equivalent head bolt models.
During engine assembly or maintenance, a bolt must be installed by torqueing it into place. Due to the head bolt's design, it has to be rotated into its slot in order to engage the threads and secure it into place. This process creates both twisting force and a vertical clamping force, which means that when the cylinders within the engine's combustion chamber begin accumulating load, the bolt will both stretch and twist. Because the bolt has to react to two different forces simultaneously, its capacity to secure the head is slightly reduced and it forms a less reliable seal in high-powered engines.
By contrast, a head stud can be tightened into place without any direct clamping force applied through the tightening. A stud can be threaded into a slot up to “finger tightness,” or the degree to which it would be tightened by hand. Afterward, the cylinder head is installed and a nut is torqued into place against the stud. The nut torque provides the clamping force, rather than the torque of the fastener itself, and the rotational force is avoided entirely. Because the stud is torqued from a relaxed state, the pressure from the nut will make it stretch only along the vertical axis without a concurrent twisting load. The result is a more evenly distributed and accurate torque load compared to that of the head bolt. This ultimately translates into higher reliability and a lower chance of head gasket failure.
One of the main differences between head bolts and head studs involves the methods used to put together 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 cylinders with near-perfect alignment. This feature makes it easier to assemble an engine using head studs. However, head bolts are far more convenient for disassembling an automobile engine or for performing maintenance, such as part replacement. Many everyday-use vehicles have master cylinders or other components that extend into the engine compartment. Under these specifications, head bolts allow the cylinders to be removed without removing the entire engine from the car, as is necessary with head studs.
In other words, head studs are better suited for high-performance vehicles with greater power requirements, while head bolts are more practical for personal, everyday automobiles. Therefore, it would be inaccurate to conclude that one type of fastener is categorically superior to the other. Rather, the preference depends on the automobile in question and the ways in which it will be put to use.