What size is a bleeder screw?

Bleeder screws, the small valves used to purge air from hydraulic lines in brakes, radiators, and other systems, do not have a universal size. The right size depends on the make, model, and system. In practice, you’ll commonly encounter metric threads in the range of M6 to M12, with pitches around 1.0 to 1.5 mm, and in some older or specialty applications, imperial sizes such as 1/8-27 or 1/4-28. The best approach is to check the vehicle’s service manual or measure the screw to be sure.

What a bleeder screw does

Bleeder screws seal a bleed nipple on a hydraulic line. When opened with a wrench, they release fluid and trapped air so the system can be refilled with fluid and pressure properly maintained. The screw includes threads that must match the component to prevent leaks and ensure a proper seal.

Applications and variations

Bleeder screws are used across brake systems, cooling systems, and other hydraulics. While many modern vehicles rely on metric thread sizes, some motorcycles and older equipment may use imperial sizes. Always verify the correct size for your specific component and year.

Common sizes you may encounter

There is no single universal size. Below is a snapshot of sizes you are likely to see across different systems, especially in automotive and motorcycle applications.

• M6 x 1.0


• M8 x 1.0


• M8 x 1.25


• M10 x 1.0


• M10 x 1.25


• M12 x 1.25


• Imperial equivalents (less common on modern vehicles): 1/4-28 UNF, 1/8-27 UNF

Note: The prevalence of each size varies by manufacturer and year. Many modern passenger cars use metric bleeder screws, while older vehicles and some motorcycle setups may use imperial threads. Always verify against official specifications or the existing screw.

How to determine the exact size for your system

To ensure you install the correct bleeder screw, use a systematic approach to identify the size rather than guessing. The steps below guide you through the process.

1. Consult the vehicle’s service manual or parts catalog for the correct bleeder screw size and type for your model and year.


2. Inspect the existing bleeder screw: measure the outer thread diameter with calipers and determine the pitch with a thread gauge. Compare measurements to standard metric (M6, M8, M10, M12) and imperial (1/4-28, 1/8-27) sizes.


3. If you cannot measure accurately, bring the screw or the component to an auto parts store or a dealership’s parts counter; they can cross-reference sizes using the VIN or model information.


4. When replacing, ensure the replacement bleeder screw has the same size thread and a compatible sealing washer or O-ring if required, and follow the torque specification in the manual.


5. After installation, check for leaks and perform the bleeding procedure as recommended by the manufacturer.

Correct sizing is essential for a proper seal and safe operation. Using the wrong size can strip the threads or cause fluid leaks, potentially compromising system performance.

Practical tips and cautions

Always take standard safety precautions when working with hydraulic systems. Use the proper wrench size to avoid rounding the head; never over-torque bleeders; and work with the system at the recommended temperature to avoid burns or damage.

Summary

Bleeder screw sizes are not standardized across all vehicles. By understanding common size ranges, verifying with manufacturer documentation, and measuring your existing screw, you can identify the correct size for your system and perform bleeding safely and effectively.

What size wrench for brake bleeder screw?

The most common size wrench for a brake bleeder valve is 10mm. However, other common sizes include 9mm and 11mm, and some domestic vehicles may use 3/8" or 7/16". A 6-point box end wrench is recommended to prevent stripping the valve. 
This video demonstrates how to use a wrench to bleed brake fluid: 58sTRQYouTube · Oct 1, 2024
Common sizes

• Metric: 10mm, 9mm, and 11mm
• SAE: 3/8" and 7/16" 

What to do

• Check the size: If possible, clean the bleeder valve and try fitting an adjustable wrench to see what size is needed before buying a specific wrench. 
• Use the right tool: A 6-point box end wrench is best to avoid rounding the valve, especially if it is corroded. 
• Be prepared for variations: Some bleeder valves may have been replaced with a non-standard size, so be prepared to use a 3/8" wrench even if you expect to use a metric one, or vice versa. 
• Use a flare nut or line wrench: These are designed to grip the valve securely and can prevent damage. 

You can watch this video to learn about different types of brake bleeder wrenches: 58sBallinatorYouTube · May 5, 2024

What size is a brake bleeder screw?

Brake bleeder screw sizes vary by vehicle, but common sizes are Metric (M10×1.0,M10×1.5,M7×1.0,M8×1.25cap M 10 cross 1.0 comma cap M 10 cross 1.5 comma cap M 7 cross 1.0 comma cap M 8 cross 1.25𝑀10×1.0,𝑀10×1.5,𝑀7×1.0,𝑀8×1.25) and Standard (SAE) (3/8"−24,7/16"−203 / 8 " minus 24 comma 7 / 16 " minus 203/8"−24,7/16"−20). The best way to determine the correct size is to physically check the screw on your specific vehicle or look up the specifications for your car's make and model. 
Common sizes

• Metric:
  • M10×1.0cap M 10 cross 1.0𝑀10×1.0 
  • M10×1.5cap M 10 cross 1.5𝑀10×1.5 
  • M7×1.0cap M 7 cross 1.0𝑀7×1.0 
  • M8×1.25cap M 8 cross 1.25𝑀8×1.25 
• Standard (SAE):
  • 3/8"−243 / 8 " minus 243/8"−24 
  • 7/16"−207 / 16 " minus 207/16"−20 

How to find the right size

• Inspect the screw: Remove the bleeder screw from the caliper and inspect it for markings. You can also measure the diameter and thread pitch. 
• Check vehicle specifications: Look up the correct specifications in your vehicle's service manual or on an auto parts store's website by entering your vehicle's make, model, and year. 
• Purchase a set: If you are unsure or working on multiple vehicles, consider buying a kit that includes a variety of common SAE and metric sizes. 

What size thread is a bleed valve?

Most standard size manual radiator bleed valves are ½”. However, they are also available in ¼” and ⅜” thread sizes.

What size is a bleeder screw on a furnace?

The Flow-Thru bleeder wrench is a must have tool for any oil burner or HVAC technician. The wrench fits standard 3/8 in. fuel pump bleeder screws.