The responsibility given to whatever cylinder head fasteners you choose to clamp your head to the surface of the block is truly a matter of mechanical life or death. Those fasteners must be able to handle the extreme abuse created within each combustion chamber as the fuel mixture burns and the pressure rapidly expands underneath, with events happening thousands of times per minute.
Summit Racing gives viewers the inside scoop on the difference between ARP cylinder head fasteners, which would be better for your engine, and how to properly install your chosen fasteners. The engineers behind these fastener designs at ARP must take into account the material selection, tensile strength, torsional strength, shear resistance and cyclic fatigue of each style of fastener to guarantee reliability in even the highest horsepower engines.
Bolt Versus Stud
When you begin your search for cylinder head fasteners, you will usually find both head studs and head bolts offered for your application. But which one should you choose? In the end it all comes down to what the specific application is being used for and how much working room you have available in the engine bay.
Simply put, head studs are installed into the block prior to the cylinder head being installed. This allows for guaranteed alignment between the cylinder head, gasket, and block, while also allowing for faster service times and more accurate values when retorquing.
The drawback is that studs are generally more expensive and require enough surrounding space to allow the cylinder head to be installed from directly overhead. So components such as brake master cylinders, brake boosters, wheel tubs, and strut bars can all become potential obstacles.
Cylinder head bolts on the other hand are installed after the gasket and cylinder head have been set in place — making it slightly more difficult to keep each piece properly aligned. This allows for installation in areas that would otherwise obstruct the installation path of the cylinder head if using studs.
One of the most important aspects when it comes to cylinder head fasteners is material stretch during installation, with two primary types of force being applied, but in vastly different ways depending on the type of fastener chosen. These two forces are measured as torsional force (twisting) and tensile force (pull/stretch).
When a cylinder head bolt is threaded into a block, there is of course the clamping force produced by the bolt threads as it is tightened, but there is also a force being applied to the bolt in a twisting action known as torsional force. This twisting action has been known to skew torque values.
As noted previously, studs are generally installed directly into the block finger tight prior to the head and gasket being set in place, eliminating the twisting force applied to the body itself during torquing. Instead, a nut is then used on top of the stud to apply the twisting action and clamping force across the entire body of the stud.
Typical tensile overload or stretch failure of a fastener exposed to too much preload [left], and a fastener failure due to torsional (twisting) shearing [right].