Engine Fasteners 101: Everything You Need to Know

Modifying stock engines has brought some interesting results. Even 4-cylinder units (like you’d find in the Subaru WRX STI) can produce double or treble the power with a little dedication and the right modifications. Often forgotten minor components like bolts and studs though are what keep all those parts together and hold the engine in place. Engine fasteners come by the dozen, and you’d be surprised as to just how many there are and the unexpected places in which they lurk. But these too also need to be replaced with performance variants if you’re serious about your mods and don’t want to blow thousands of dollars’ worth of parts to smithereens.

What are Engine Fasteners? 

Engine fasteners are the bolts, studs and nuts that are used in the assembly of various engine parts and keep the whole engine together. They have a crucial role in ensuring that every component stays in place and works as it should. While stock fasteners are rarely replaced in production cars (and should last the lifetime of the vehicle), it’s when upping horsepower numbers with performance parts and engine rebuilds that fasteners are replaced. They minimise the risk of leaks, vibrations, excessive noise and in some cases, component or engine failure. 

Types of Fasteners

Fasteners come in different shapes and sizes and can be divided into engine studs, bolts, and nuts. Each has a specific role and secures different parts.

Engine Bolts

Bolts are fasteners with external threading and are inserted in component holes. They also have an integrated head (usually a hex design) to make tightening and torquing the fastener easier and quicker, with or without a compatible nut. Some main types include those used in engine mounts to hold the engine in place, connecting rod bolts used in conrod caps, and head bolts found in the cylinder head and used to secure the engine head gasket. Rod bolts are some of the most stressed engine parts and are some of the most likely to fail. 

Studs 

These are externally threaded fasteners, either along the whole length or partially, often with unthreaded mid-sections. They differ from bolts in that they lack heads. Studs resemble threaded bars and are often used to join other components with internal threading. You’ll find studs taking the role of guide pins in various components and engine covers when affixing gaskets and more. Another use of engine studs is replacing bolts in high-stressed areas, specifically when increasing power and torque. To achieve the required clamping force, they may also be supplied with compatible washers. 

Nuts 

These are internally threaded engine fasteners and are often used along with bolts or studs to provide increased clamping force. Most engine parts though forego the need for nuts, instead relying on tapped holes to secure fasteners internally. 

Bolts vs Studs – Which to Use in Engine Rebuilds?

Studs are praised for their high strength and the ability to resist extreme pressure. That’s why they’re preferred for engine modifications or complete rebuilds instead of bolts. A large part of this is down to how they’re made. Performance studs use higher-grade alloys and undergo extensive treatment. They’re also easier to install, so in engines with frequent servicing needs, like in racing applications, they’re the better choice. What’s more, they apply more clamping force to head gaskets, and cylinder heads, and more precision in alignment with higher torque potential. And lastly, unlike bolts, they can be reused several times without the risk of failure. So if you’re thinking of bolting on a bigger turbo or changing to forged internals, then studs are the way to go. 

Choosing the Right Fasteners for the Application 

Materials, and by extension, overall clamping force and tensile strength are the main factors to look at when choosing engine fasteners. Understandably, you’ll also need bolts or studs in the right sizes for the intended application. 

Engine fastener materials

Stainless steel has a high tensile strength (170,000 psi), is heat resistant and performs well against corrosion. Low grades are seen in accessory bolts and studs, while higher-grade alloys with added trace metals are seen in connecting rod bolts. 
Move up to chromoly steel for more tensile strength (up to 200,000 psi) and engine studs and bolts that can handle mild racing. The material has high fatigue strength so won’t crack or deform under heavier loads. Chromoly connects rod bolts and head and main studs and bolts. Upgraded versions, known as ARP2000 add roughly 10 per cent higher strength, ideal for applications like drag racing. 
Titanium is preferred for its superior corrosion resistance and very low weight (about half that of steel) while still being able to maintain high pressures of 180,000 psi. Choose Inconel, a nickel-based alloy for the highest fatigue, heat and corrosion resistance and exceptionally high strength. A race-specific Inconel variant, ARP3.5, is used in connecting rods found in Supercars and Formula 1.  

Sizing Fasteners

Besides head, main and rod studs and bolts, there are also fasteners used in intake manifolds, flywheels, oil pans, carburettors and more. Each comes in different sizes. This means differences in lengths, diameters, and thread pitch for the application. Manufacturers will specify where the fasteners are used, and with which vehicles and engines they’re compatible, so make purchases and replacements easier.