What Are Bolts with Holes in the Head and When Should You Use Them?

What Are the Main Types of Bolts with Holes in the Head?

Not all bolts with head holes are the same. The design of the head and the hole’s purpose varies depending on the application.

Eye Bolts

Eye bolts are the most recognizable type. Instead of a standard head, they feature a closed loop, or “eye,” at the top. This eye is designed for attaching hooks, ropes, or chains, making them essential for lifting and rigging applications. They are typically made from carbon steel, stainless steel, or alloy steel, with load capacities ranging from a few hundred pounds to over 10,000 pounds. Choosing the correct load capacity is critical for safety.

Hexagon Head Bolts with Drilled Holes

These look like standard hex head bolts but have one or two holes drilled through the head, either across the flats or through the corners. These holes are sized to accept cotter pins or locking wire. This design is extremely common in high-vibration environments. For instance, a 2024 report by the Automotive Fasteners Association noted that 85% of engine mounting bolts use this design to prevent loosening during operation.

T-Head Bolts with Holes

T-head bolts have a T-shaped head designed to fit into a T-slot, allowing for quick installation and adjustment without the bolt rotating. When the head is drilled with a hole, it can accept a spring clip or other locking device to keep the bolt securely positioned within the slot. They are often used on machine tool tables and conveyor systems.

Round Head Bolts with Holes

These bolts have a smooth, domed head that minimizes snagging. A hole drilled through the center allows for a cotter pin. They are frequently used in marine hardware and outdoor equipment, where the rounded shape helps shed water and the locking pin ensures the nut stays tight even under constant motion and vibration.

When Should You Use Bolts with Holes in the Head?

These are not general-purpose fasteners. They are a necessity in specific, demanding situations where a standard bolt would be a point of failure.

High-Vibration Environments

Vibration is the enemy of threaded fasteners. It can cause a standard nut to spin off over time. Bolts with head holes solve this by allowing a cotter pin or safety wire to physically lock the nut in place. In an industrial pump running at 3,000 RPM, a hex head bolt with a drilled hole and cotter pin is far less likely to fail. Studies have shown that using these bolts can reduce fastener failure in high-vibration settings by up to 60%.

Lifting and Rigging

When lifting a heavy load, the connection point must be absolutely secure. Eye bolts are the standard solution. The closed eye ensures that a hook or shackle cannot accidentally detach. It is critical to select the right type of eye bolt. A “shouldered” eye bolt can handle angled lifts, while a non-shouldered type should only be used for straight vertical lifts.

Safety-Critical Equipment

In industries like aerospace, a fastener failure is not an option. Bolts with head holes are used to create safety wire locking systems. Mechanics weave stainless steel wire through the holes of a series of bolts, twisting it to create tension. If one bolt tries to loosen, the wire locks it in place. The Federal Aviation Administration (FAA) mandates this practice for many critical aircraft components.

Outdoor and Corrosive Environments

In marine or outdoor settings, bolts face constant moisture and movement. A round head bolt with a hole, made from corrosion-resistant stainless steel, is an ideal choice. The smooth head sheds water, reducing corrosion, and the cotter pin provides mechanical security even if the threads begin to corrode.

How to Choose the Right Bolts with Holes in the Head

Selecting the wrong bolt can lead to project failure or create a safety hazard. A systematic approach is essential.

Step 1: Identify Your Application’s Requirements

Start by defining the primary function of the bolt.

• Is it for lifting a load? If yes, you need an eye bolt. You must know the weight and angle of the lift.
• Is it for securing a part against vibration? If yes, you likely need a hex head bolt with drilled holes.
• Is it for an outdoor application? If yes, you need a corrosion-resistant material, like stainless steel, and a head shape suited to the environment.

Step 2: Determine the Correct Size and Thread

Measure the hole diameter accurately, preferably with a caliper. The bolt should fit snugly, not loosely.

• Coarse threads (UNC) are generally better for softer materials like wood or plastic, as they provide more grip.
• Fine threads (UNF) are better for hard metals and applications requiring precise adjustments, such as in automotive engines.

Step 3: Select the Right Material

The material determines the bolt’s strength and corrosion resistance.

Step 4: Check for Industry Standards

Reputable bolts will be marked with their grade and compliance to standards. These markings are your assurance of quality.

• ANSI/ASME: Common in North America, ensures dimensional accuracy.
• ISO: The international standard for consistent performance.
• SAE: Often seen on automotive fasteners. For example, SAE J429 Grade 8 indicates a very high-strength alloy steel bolt.

Installation Tips: How to Use Them Properly

Even a high-quality bolt will fail if installed incorrectly. Follow these best practices.

Prepare the Surface

Ensure the mounting surface is clean, flat, and free of debris like dirt, rust, or paint. An uneven surface can create stress points that weaken the bolt.

Use the Right Tools

Always use the correct tool for the bolt head. For a hex head, use a socket wrench, not an adjustable wrench, to get a solid, even grip. For an eye bolt, use a wrench on the square shank just below the eye. Never twist the eye itself to tighten, as this can distort it and reduce its load capacity. Crucially, use a torque wrench to tighten the bolt to the manufacturer’s specified value.

Install the Locking Device Correctly

The hole is useless without the correct locking device.

• For Cotter Pins: Insert the pin through the hole in the bolt and the slot in a castellated nut. Bend the ends of the pin outward, at least 90 degrees, to lock it.
• For Lock Wire: Weave the wire through the holes of a series of bolts, pulling it tight and twisting it to create tension. This creates a chain that prevents any single bolt from loosening.
• For Safety Clips: Slide the clip into the hole and bend the ends to grip the bolt head. This is a quick option for applications needing frequent adjustment.

Inspect Regularly

In critical applications, inspect the bolts and locking devices regularly. Look for signs of wear, corrosion, or a broken cotter pin. In an industrial setting, a 3-month inspection cycle is a good practice. In a marine environment, monthly checks are wise.

How to write a spec that gets priced correctly

A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.

• The application and the environment it runs in
• The standard the part must meet
• Process and material that can meet it
• Inspection and documentation package
• Volume and ramp schedule

The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.

What actually drives the price

Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:

• process selection
• material and certification
• tolerance band
• inspection and documentation
• volume

Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.

Problems you will meet, and the fix

Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.

• A consumer-grade process chosen for an industrial duty
• Documentation that arrives after the parts
• A tolerance tighter than the application needs
• No plan for the second source

Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.

What to check on the first article

The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.

• Every dimension on the drawing, with the measured value written beside it.
• The material certificate, matched to the heat or lot number on the parts.
• The finish against the sample you approved, under the same light.
• Fit against the mating part, assembled rather than measured.
• Packaging, because parts that arrive damaged were never in tolerance.

Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.

How to compare two quotes fairly

Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.

Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.

Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.

Scaling from one part to a batch

The first part teaches you the process. The hundredth teaches you the process cost.

Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.

Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.

How to read a supplier's quote

A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.

• The scope line. It should name the process, the material and the finish, not just the part.
• The tolerance basis. General tolerance or the drawing, and which standard it follows.
• The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
• The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.

Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.

What to send with the first order

The first order sets the pattern for every order after it. Send a complete package once and the shop files it, and the second order takes a fraction of the time.

• A STEP or native model plus a PDF drawing carrying the same revision letter.
• The material callout by standard number, not by trade name.
• The finish and the exact faces it applies to.
• Any test the part must pass: fit, seal, drop, salt spray, dielectric.
• A contact who can answer a question inside one working day.

Then write down what was agreed. An email that confirms the material, the finish and the date is worth more than a longer purchase order that leaves them open.

Real-World Case Study: Preventing a Factory Shutdown

A manufacturing plant in Ohio was experiencing frequent jams in its main conveyor system. The problem was traced to the bolts holding the drive pulley, which were loosening due to constant vibration from a 50-horsepower motor. The standard hex bolts originally used were failing.

The maintenance team replaced them with ½-inch hex head bolts with drilled holes (SAE J429 Grade 8). They installed castellated nuts and secured them with cotter pins, torquing each bolt to the recommended 75 foot-pounds.

After the change, the conveyor ran smoothly for six months without a single vibration-related issue. The plant manager estimated that this simple switch to the correct fastener saved them $20,000 in downtime and repair costs.

Bolts with holes in their heads are a simple but highly effective solution for applications where standard fasteners are not enough. By providing a means to physically lock the nut in place, they offer critical protection against vibration, ensure safety in lifting operations, and provide the redundancy required in high-stakes industries like aerospace. The key to success is selecting the right type of bolt for your specific application—considering the environment, load, and need for security—and installing it correctly with the appropriate locking device and torque.

How we can help

YiguProto runs contract manufacturing in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.

Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.

Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our contract manufacturing. You can also talk to an engineer before you commit to a drawing.

Frequently asked questions

The short version

Good parts reward the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.

Tighten only what mates. Test a sample before the run. Then let the process do the rest.