A Quick Guide on the Types of Bolts, Studs, and Nuts

Your car and its engine rely on various fasteners to attach different parts to the chassis, keep them from getting loose, and dampen vibrations while running. There are different types of fasteners, each with their unique characteristics, strengths, and weaknesses. Some of them work together, while others are incompatible.

This quick guide looks at the various nuts, bolts, studs, and other engine fasteners in your car’s engine block.

Types of Bolts and Nuts in the Engine

Engine Bolts

Bolts are externally threaded fasteners. Their thread is on the outer side of their rod. Calling them blunt screws isn’t far from the truth–the border between bolts and screws is fuzzy at best.

One of the distinguishing parts of engine bolts is their head, the topmost part that acts as the surface gripped by the fastening tool. For example, hex head bolts have six-sided heads. Meanwhile, 12-point bolts have a dozen equally spaced points on their head in the form of a star.

Some of the most critical engine fasteners are bolts. Head bolts attach the cylinder head to the engine, and intake manifold bolts perform the same job for the intake manifold. Meanwhile, rod bolts secure the connecting rod and the crankshaft, while connecting rod bolts fasten the same mechanical linkage to the cylinder piston.

Head, rod, and connecting bolts are not interchangeable. If you have to replace the fasteners, use exact replacement and avoid substitutes.

All of these bolts and fasteners should be replaced with exact replacements, and are not interchangeable.

–Anthony Harlin, ASE Certified Master Automobile Technician

Engine Nut and Washer

In many ways, nuts are the opposite of bolts. They are internally threaded fasteners that receive bolts. Their thread is on the internal side and matches the one on their bolt counterpart.

Nuts usually have washers with them. A washer is a narrow, level disc between the nut’s surface and the bottom of the bolt’s head. It evenly distributes the load of the threaded fastener on the receiver and can serve as a spacer to allow longer bolts. Some washers can also dampen vibrations or seal the fasteners against liquids like water.

Engine nuts and washers often work together with bolts. The combination of two different fasteners makes the connection tighter and more stable.

Some nuts feature a nylon ring that locks the fastener down when you tighten the component. These are called lock nuts. If you replace an engine nut, check if it’s a nylon nut and use the appropriate, identical fastener. 

Engine Studs

Finally, we have studs, which are externally threaded rods that enter internally threaded holes. After a stud goes into its hole, a nut usually fastens on it for additional security.

Studs serve in heavy-duty jobs that would break other fasteners. For example, main studs attach the main bearing caps to the engine block. The main bearings ensure the crankshaft stays in place and rotates freely in the engine block. Given the importance of these parts, the main studs must have the strength to support the weight and withstand the immense force imparted by the rapidly spinning crankshaft.

In addition to the main bearing caps, engine studs secure other vital engine parts like belt drives, carburetors, distributor clamps, oil pans, and timing chain covers. Some valve covers also use studs.

When Should You Replace Engine Fasteners?

Generally, there are two reasons for you to replace engine fasteners. The first is corrosion, damage, or wear that weakens or breaks the fasteners. The second is the need for more durable connectors when installing new engine parts.

Repair and Replacement

Engine fasteners are sturdy but not indestructible. Sure, the rustproof material they’re made of can take a beating, but they still get worn down by everyday use.

Sufficiently sharp shocks or numerous lesser jolts over time can crack or deform nuts, bolts, or studs. If the fastener’s anti-corrosion coating breaks, rust can form on its surface, further weakening it and potentially trapping it within its hole.

A damaged, rusted, or worn engine fastener cannot keep parts fastened together as effectively as a new one. It can lead to issues like increased vibrations and louder noises. In the worst-case scenario, the fastener and the engine part might fail.

Engine Upgrade and Customization

Do you want to squeeze more performance out of your car’s engine? There are many ways to boost its performance, such as replacing factory-issue parts with high-performance equivalents and installing products that enhance efficiency and power generation.

These modifications generally increase the engine’s overall weight. Furthermore, high-performance parts usually put more strain on the standard fasteners that attach them to the engine block. For example, the new components might generate more heat or vibration than what the factory-issue nuts, bolts, and studs can handle.

If you upgrade your engine, you usually need to install stronger nuts, bolts, and studs with higher stress tolerances.

Choosing the Right Engine Fastener

Whether you replace faulty engine fasteners or upgrade them alongside the rest of the engine, you want the best part for the job. Usually, the choice boils down to either bolts or studs. Both connectors have advantages and disadvantages, and one might prove more suitable than the other for certain applications.

To get a better idea, let’s directly compare cylinder head bolts and cylinder head studs:

Convenience

Many street-legal vehicles have engines whose master cylinders stick into their engine compartment. With these cars, it’s more convenient to use engine fasteners that enable the removal of the cylinder heads with the engine in the car.

Engine Assembly

A racing engine pays for its high performance with increased maintenance. It must undergo servicing and repair more frequently. At the same time, it must be serviced as fast as possible.

Engine studs make it easier and faster to disassemble and reassemble an engine block. You can quickly take it apart, replace the faulty part, and assemble it again.

Another advantage of engine studs over head bolts is superior strength. Studs are stronger than bolts, and replacing the head bolts with ARP head studs is a significant upgrade. 

Parts Alignment

Studs also make it easier to align the cylinder head and gasket. These parts must be properly aligned, or else they might leak or blow.

Torque Loading

Torque loading refers to the application of force acting over a radial distance to rotate an object. In practical applications such as fasteners, it ensures two surfaces press together in the proper and efficient manner.

Both bolts and studs provide torque loading. However, studs have better precision and consistency.

Choosing the Right Engine Fastener Head

The head design of the engine fastener can also determine its suitability. Hex head fasteners don’t require specialized socket bits in locations that interfere with a wrench or require proper torque. In comparison, 12-point head fasteners are more accessible using socket wrenches.

Properly Tightening Engine Fasteners

Tightening an engine bolt or stud takes more than twisting the fastener until it refuses to budge. You have to strike the right balance to achieve the correct clamp load.

If you apply insufficient torque to the fastener, its clamp load can’t maintain the connection between the parts it joins together. However, putting excessive amounts of torque might stretch the connector too much. Both issues can lead to the failure of the component.

You can measure the tension applied to an engine fastener with three methods. You can use a conventional torque wrench, a stretch gauge, or a torque angle meter.

Whatever you plan to do with the engine of your car, you need engine fasteners that can handle the load and stress of your driving habits.

Get New Car Bolts Delivered to Your Home

Has one or more of your car’s bolts developed issues, such as corrosion or physical damage, that affect their durability and tightness? It’s best to stop driving your vehicle to minimize the risk of additional problems. Luckily, CarParts.com can simplify the repair job by delivering replacement bolts to your doorstep.

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