Brake Line Thread Identification Guide: Sizes, Charts, and Tips

Understanding brake line thread identification is crucial for ensuring the safety and performance of your braking system. In this guide, we at MFCP will provide you with comprehensive information on brake line sizes, fitting identification, and measurement techniques. Whether you're dealing with metric or imperial brake lines, this guide covers everything you need to know.

What are Brake Lines?

Brake lines are essential components of your vehicle's braking system, responsible for transmitting hydraulic pressure from the master cylinder to the brake calipers. This hydraulic pressure is what activates the brakes, allowing your vehicle to slow down or stop. Proper identification and sizing of brake lines are critical for maintaining braking efficiency and safety. Incorrect sizing can lead to brake failure, which can be dangerous.

Common Brake Line Sizes

Brake lines come in various sizes, commonly measured in inches or millimeters. Knowing the correct size for your brake lines is important for ensuring compatibility with your vehicle's brake system. Here are some standard sizes:

• 3/16" (4.75mm): This is the most common size used in most passenger vehicles. It is often referred to as the "universal size" for brake lines.
• 1/4" (6mm): Typically found in trucks and larger vehicles, this size can handle higher pressure and is used in applications where the braking system requires more robust lines.
• Metric Sizes: Metric sizes like 4.75mm, 6mm, and 8mm are commonly used in European and some Asian vehicles. It's important to use the correct metric size to avoid issues with threading and sealing.

There are two systems for measuring threads: metric and imperial (same as SAE or American). Metric thread is measured in millimeters. For example, M10x1 refers to 10mm diameter and 1mm thread pitch (M stands for Metric). Imperial thread diameter is measured in fractions of an inch, and imperial pitch is measured in threads per inch (TPI). For example, 3/8” x 24NF refers to 3/8” diameter and 24 threads per inch pitch (NF stands for National Fine; other options are NS: National Special, and BSF: British Standard Fine).

Example:

How to Measure Brake Line Size

Accurately measuring the brake line size is vital for selecting the right fittings and ensuring a secure connection. Here’s a step-by-step guide:

1. Gather Tools: You’ll need a caliper to measure the outer diameter of the brake line, a thread gauge to measure the thread pitch, and a reference chart for comparing your measurements.
2. Measure Outer Diameter (OD): Use the caliper to measure the OD of the brake line. Make sure to measure at multiple points to get an accurate reading, as the line can sometimes be slightly deformed.
3. Identify Thread Size: Use the thread gauge to measure the thread pitch and size. The thread pitch is the distance between threads, measured in threads per inch (TPI) for imperial sizes or in millimeters for metric sizes.
4. Compare with Chart: Refer to a brake line size chart to confirm the measurements. This will help you identify the correct size and type of brake line and fitting you need.

Alternatively, you can take an unknown male or female part and try to screw it together with a part you already know the size of – if it fits, you now know the size of both.

However, you do not need a thread ID kit or identified nuts and bolts for correct thread identification. You can measure the thread to identify it.

1. Use calipers to measure the thread diameter. The male threads will be slightly smaller than the nominal size, and the female threads will be slightly larger than their nominal size.
2. Measure the pitch. For imperial sizes, count the number of crests within a ½” section and multiply it by two to get TPI. If your thread is long enough, count the number of crests within one inch to get TPI. For metric sizes, just measure the distance between the two crests.

Brake Line Size Charts

For quick reference, use these brake line size charts to identify the correct size and thread. These charts are essential tools for anyone working with brake lines. They allow you to quickly and accurately identify the correct size, reducing the risk of using incompatible components.

Imperial Sizes:

Types of Brake Line Fittings

There are several types of brake line fittings, each designed for specific applications. Understanding the differences between these fittings is crucial for selecting the right type for your vehicle:

• Double Flare: Used in most automotive applications, double flare fittings are known for their durability and secure fit. The end of the brake line is flared twice, creating a strong seal that can withstand high pressure.
• Bubble Flare: Common in European vehicles, bubble flare fittings have a single flare at the end of the line. They are easier to install but may not be as durable as double-flare fittings.
• AN (Army-Navy) Fittings: Used in high-performance applications, AN fittings are designed to withstand extreme conditions. They have a 37-degree flare and are commonly used in racing and military vehicles.

Thread Identification Tips

1. The most common threads are: 3/8 x 24NF, 7/16 x 24NS, and M10 x 1
2. The most commonly confused sizes are 3/8” with 10 mm, and 7/16” with 11 mm. The 3/8” x 24NF will screw into M10 x 1 thread but not the other way around, so always try to screw M10 x 1 into 3/8” x 24NF to see if you have a match. If you are deciding between 7/16” and 11mm, the safer bet is to go with 7/16” because 11 mm is extremely rare.
3. It is possible to have a metric thread on one end of a brake line, and an imperial thread on the other end.
4. You can generally distinguish whether you have a metric or imperial threaded brake line from the year and origin of your vehicle:

• • European (not UK) – Metric (99%)
  • Asian – Metric (99%)
  • UK up to 1976 – Imperial
  • UK after 1976 – Metric
  • US prior to 1980 – Imperial
  • US after 1980 – Either Imperial or Metric

In Conclusion

Proper brake line identification and measurement are crucial for maintaining your vehicle's braking system. By following the steps outlined in this guide, you can ensure that your brake lines are correctly sized and fitted, enhancing safety and performance. Remember to always use high-quality components and follow the manufacturer's recommendations for installation and maintenance. For more information, visit MFCP or contact our experts for assistance.

 Posted by Marek Bobik & Vincent Lockwood