Ever wondered what to call that familiar piece of hardware—a bolt with a hole in the middle? Whether you’ve spotted one during a DIY project or are troubleshooting machinery, understanding these specialized bolts can save you time and hassle.
Knowing the purpose and name of a bolt with a hole isn’t just trivia—it helps you pick the right part for your next repair or build. This article unveils the answer, explores common uses, and shares quick tips for choosing the perfect bolt.
Related Video
What Is a Bolt With a Hole in the Middle?
A “bolt with a hole in the middle” is typically a standard bolt that features a drilled hole through its shank or, less commonly, a hollow core running its full length. This hole can serve several purposes, including enhanced security, lubrication, safety wiring, or even specialized fluid passageways. These bolts are often called “drilled bolts,” “safety wire bolts,” or, if hollow, “hollow bolts” or “banjo bolts” in some applications.
Let’s break down everything you need to know about these bolts: how they’re made, why they’re used, and tips for choosing and installing them.
Understanding Bolts with Holes
Key Types
- Drilled Heel or Shank Bolts: Solid bolts with a small crosswise hole drilled through the head or shank for safety wiring.
- Hollow-Core Bolts: Bolts manufactured with a hollow center passage, often used to allow fluid or gas to pass through (like banjo bolts).
- Torx or Security Fasteners: Screws or bolts featuring a recess in the head (sometimes described as a “hole” in the driving area) that requires specialized tools.
Why Add a Hole?
The main purposes for a bolt with a drilled or hollow section include:
- Safety wiring: Preventing critical fasteners from loosening under vibration.
- Lubrication: Allowing grease or oil to be delivered deeper into machinery.
- Fluid/Gas transport: Used in hydraulic, braking, or lubrication systems.
- Security: Implemented in tamper-resistant fasteners.
- Weight reduction: For specialized industries like aerospace.
How Are Bolts With Holes Made?
The process depends on the specific design and intended use. Here are the two common methods:
1. Drilling After Manufacturing
- A standard bolt is manufactured as normal.
- A precise, small-diameter hole is drilled transversely through the bolt head, or longitudinally through the shank, as required.
- This is typical for safety wire bolts or for grease passageways.
2. Specialized Manufacturing
- The bolt is forged or machined with a hollow center from the start (for banjo bolts or hollow hydraulic bolts).
- Tolerances and alloy selection matter more here, as the integrity of the fastener is critical despite the removed material.
Practical Steps if You Want to Make Your Own
If you ever need to add a hole to a bolt for a DIY application (such as safety wiring):
- Select an Appropriate Bolt: Start with a high-quality bolt, strong enough to withstand your load even if drilled.
- Mark the Drilling Location: For a safety wire, most people drill just below the head or through a pre-existing flat area.
- Clamp Securely: Secure the bolt in a vise, protecting the threads.
- Choose the Correct Drill Bit: Pick a small diameter bit. Carbide or cobalt bits are best for tough steel.
- Drill Slowly: Use cutting oil to keep the bit cool and ensure a straight, clean hole.
- Deburr: Remove any sharp edges with a deburring tool or file.
- Clean Thoroughly: Ensure no metal shavings are left behind.
Note: Drilling bolts yourself can weaken them if not done correctly. When in doubt, purchase professionally made drilled bolts.
Applications: Where Are These Bolts Used?
Bolts with holes in the middle show up in many industries:
- Automotive and Motorsport: For safety wiring on brake calipers or suspension parts.
- Aerospace: On flight-critical fasteners to guarantee nothing comes undone in vibration-prone environments.
- Industrial Machinery: For lubrication through hollow bolts.
- Hydraulic Systems: Banjo bolts serve as bolt-like connectors with a fluid passageway.
- Security Applications: Tamper-resistant fasteners in electronics or public installations.
Let’s look at some more details.
Safety-Wire Drilled Bolts
- Used extensively in aviation, racing, or heavy equipment.
- Require special safety wire to be threaded through after installation.
- Prevent catastrophic loosening.
Banjo Bolts
- Hollow bolts with a cross-drilled hole used with a banjo fitting.
- Allow hydraulic fluid or oil to pass through, commonly found in brake or clutch systems.
Security Bolts (with Recessed Heads)
- The “hole” in the middle refers to the indentation required for a specialized bit.
- Used when tamper-resistance is needed.
Benefits of Using Bolts With Holes
- Enhanced Safety: By allowing safety wiring, bolts cannot back out due to vibration.
- Improved Reliability: Hollow bolts for lubrication or fluid increase machinery lifespan.
- Security: Tamper-resistant bolts deter unauthorized access.
- Special Functionality: Enable use in specialized industries with stringent demands.
Challenges and Considerations
While bolts with holes offer many advantages, they also require some caution.
- Reduced Strength: Drilling removes material, so only drill where it’s structurally acceptable.
- Precision Required: Poorly drilled holes can weaken or damage the bolt.
- Specialized Installation: Safety wiring and hydraulic banjo bolts must be installed correctly.
- Increased Cost: Specialized or custom drilling adds to the cost.
- Limited Reusability: Hollow core or drilled bolts in high-stress environments may wear faster.
Expert Tip: Always check manufacturer recommendations for torque and reusability, especially in safety-critical applications.
Best Practices for Selecting and Using Drilled or Hollow Bolts
- Understand Your Application
- Identify if safety, lubrication, fluid transfer, or security drives your need.
- Choose the Right Material
- Use high-grade steel, stainless, or alloy, especially if drilling after purchase.
- Consider Professional Supply
- Whenever possible, buy pre-drilled or professionally manufactured bolts.
- Follow Torque Specs
- Follow factory torque settings, especially if using safety wiring.
- Inspect Regularly
- In high-vibration environments, check that wiring and bolts remain secure.
- Use the Correct Safety Wire and Tools
- For safety wiring, use proper stainless wire and safety wire pliers.
- Avoid Over-tightening
- Hollow and cross-drilled bolts may be more prone to shearing if over-torqued.
Cost Tips — Saving Money
Bolts with holes cost more than standard bolts due to extra processing or manufacturing steps. Here’s how to keep costs manageable:
- Bulk Purchase: If you use many, buy in quantity for price breaks.
- Choose Standard Sizes: Custom lengths/diameters add cost.
- Shipping Optimization: Order from suppliers who bundle shipping or have local distribution.
- On-Site Customization: If you have precision drilling equipment and expertise, you may economically drill bolts in-house for non-critical uses—but remember the strength implications.
- Stock Only Essentials: For expensive types like banjo bolts, keep only critical spares on hand.
Related Domains and Concepts
- Fastener Terminology: Knowing your terms — for example, “drilled shank,” “banjo bolt,” “safety wire hole”— helps when ordering.
- Torx Security: Tools designed for fasteners with center holes for anti-tamper applications.
- Engineering Guidelines: Always follow minimum edge distances if you’re drilling holes near a bolt’s edge or installing new fastened joints.
- Hardware Suppliers: Some suppliers specialize in safety-wire-ready bolts, banjo bolts, and others specifically designed for unique industry needs.
Frequently Asked Questions (FAQs)
1. What is the main purpose of a bolt with a hole in the middle?
The main purpose depends on the hole’s location and design. Most commonly, drilled holes allow safety wire installation, which prevents bolts from loosening due to vibration or movement. Hollow bolts (banjo bolts) permit fluids or gasses to pass through in hydraulic and lubrication systems.
2. Is it safe to drill a hole through a standard bolt myself?
It’s possible but requires caution. Drilling can weaken the bolt, especially if done improperly. For critical or high-stress applications (like automotive or aviation), it’s best to buy bolts designed and manufactured with holes, ensuring correct material and strength.
3. How do I use safety wire with a drilled bolt?
After installing the bolt to its correct torque, thread stainless safety wire through the drilled hole and twist the ends together so the wire is pulled tight. The wire should be anchored to a fixed point or another bolt to prevent rotation, ensuring the bolt can’t back out.
4. What is a banjo bolt and where is it used?
A banjo bolt is a hollow bolt with a hole through its shank and a cross-drilled hole. It’s commonly used in hydraulic brake and clutch systems to transfer fluid through a banjo fitting, allowing both a secure connection and fluid passage.
5. Are bolts with holes more expensive or harder to find?
Yes, bolts featuring drilled or hollow sections are typically more expensive and less readily available than standard bolts due to the specialized manufacturing needed. For most industrial, automotive, and aerospace uses, these bolts are stocked by specialty fastener suppliers.
In Summary
Bolts with holes in the middle may seem like a small tweak but play a crucial role in safety, fluid management, and security across industries. Whether you need to safety wire a critical fastener, allow lubrication into machinery, move hydraulic fluids, or deter tampering, there’s a bolt designed for your needs.
Choose high-quality, application-specific bolts—preferably from reputable suppliers—for the best peace of mind. Use proper tools, follow best practices, and always consider the unique stresses and requirements of your application. With bolts like these, a little added complexity delivers a lot more reliability and safety.