The nut torque provides the clamping force, rather than the torque of the fastener itself, and the rotational force is avoided entirely. Afterward, the cylinder head is installed and a nut is torqued into place against the stud. A stud can be threaded into a slot up to “finger tightness,” or the degree to which it would be tightened by hand. 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.īy contrast, a head stud can be tightened into place without any direct clamping force applied through the tightening. 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. 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. Torque Efficiencyĭuring engine assembly or maintenance, a bolt must be installed by torqueing it into place. Some head studs are designed with gasket and cylinder alignment in mind, and they are typically more expensive than the equivalent head bolt models. 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.
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Studs are carefully shaped to make them concentric, usually requiring a series of cuts to produce a straight, balanced component.
Like bolts, head studs are also thermally treated at high pressure to improve structural strength. Some manufacturers produce head bolts with wide flange dimensions, which reduce the need for removing the bolts or the valvetrain when conducting cylinder maintenance. They provide relatively high strength and durability, and frequently come packaged with hardened washers or nuts. 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. Engine Head Fastener DesignĬomparatively speaking, head bolt design tends to focus on stability and convenience. For example, a high-end or racing model car will have drastically different engine fastener requirements than a vehicle designed for everyday purposes. The power and acceleration potential of an engine often dictates the type of head fastener that will be used. Ease of engine assembly and disassembly can be a significant factor, as well as torque pressure, gasket alignment, and overall engine performance. When evaluating the various benefits of studs and bolts, it is helpful to keep a number of considerations in mind.
In addition, studs do not have a head, whereas bolts always do. Head Studs vs Head Boltsįor 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. 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. 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.