What are head studs? A head stud is a fastener used to attach the engine head.
Head Studs are used in engines to either secure an engine with two mating parts or as a fastener for one part of an engine. There are typically three types of head studs, these being pressed-in studs, machined-threaded studs, or female threaded-. Studs with circumferential grooves can provide better sealing properties than those without them; they also tend to be more expensive. If the threads on the threads form a ridge (i.e. if they are not “fine-pitch”) then the studs are known as “ribbed studs”, or occasionally as “taper shank studs”.
Their function is to locate the cylinder head onto the block. When fastened, head studs prevent the cylinder head from moving up and down relative to the block and also prevent it from rotating around its axis. Although located in individual engine blocks, these are essentially bolts. This is because their function is to provide a stronghold between two separate pieces of metal, rather than attaching one piece of metal to another piece. Two main factors limit their ability to transfer forces. Firstly, head studs are usually made of an alloy called ‘Fe7Ni5Al7Co’ similar to that used in engine blocks. Secondly, they are subject to the same basic laws of physics that apply to all fasteners.
This means that while they are designed to be able to resist some forces, often more than the tolerances allowed for in their design specification, they are not designed to do so under all circumstances. For example, if a head stud is subjected to a force strong enough to cause it to fail under its design conditions, then more force may be applied before it fails. This may allow further damage and the possibility of catastrophic failure (which could result in engine failure).
There are two main standards for head studs: those used in General Motors engines and those used in Toyota engines. These standards differ across a number of dimensions (such as the distance between the holes).
Welding studs to make a ‘stud block’ is a less common method, and welding heads to the block is even less common owing to the high temperatures involved.
The engine head usually consists of two pieces: an upper and a lower (the former is also known as a “deck”). The upper piece forms part of the cylinder head and may contain intake and exhaust ports, coolant passages, spark plug sockets, rocker arms, etc. It is secured to the engine block by usually three studs, two of which are longer than the third (known as “center stud”). The center stud goes through the engine block and is sandwiched between the upper and lower pieces; this allows it to be located off-center in order to give greater rigidity to the head.
The main failure mechanism for head studs is fatigue failure. This can occur either at room temperature or under elevated temperatures (e.g. when ‘memory’ occurs). This type of failure commonly leads to a sudden breakage that can lead to a cylinder head becoming detached from an engine block, which could cause catastrophic loss of cooling, extreme stress on other parts of an engine, and possibly even engine collapse. The risk of failure increases as the engine ages and as the load (in terms of engine torque and speed) applied to the engine increases.
This is why it is important to keep an eye out for any signs of failure; if an engine fails due to a stud breaking, then there is a much greater chance that it will be able to continue running before catastrophic failure occurs. There are several things that people should look for in order to check on their own engines.
Aside from studs, the head also includes the camshaft gearbox, timing gearbox, distributor gearbox or electronic control module (ECM), and intake manifold. Although they are often called “heads”, they are really only one component.