How to Measure a Broken Bolt and Find a Replacement

{markdownData.frontmatter.authors} Picture @adamtester    Sat, 16 Dec 2023 16:56:00 GMT

How to Measure Bolt Size with Common Tools

Car maintenance often involves dealing with various types of bolts, and it's not uncommon to encounter broken bolts during repairs or modifications. Knowing how to accurately measure a bolt size is essential for finding the right replacement. This guide will walk you through the process of measuring bolt sizes using common tools, and also delve into the comparison between steel and stainless-steel bolts for automotive use.

Identifying and Measuring Bolt Sizes

Tools You'll Need

  • A ruler or a measuring tape
  • A caliper (optional, for more precise measurements)
  • A thread pitch gauge

Steps to Measure Bolt Size

  1. Determine the Diameter:

    • Place the ruler or measuring tape across the bolt's shank (the non-threaded part).
    • Measure the diameter, which is the widest part of the shank. If you have a caliper, use it for a more precise measurement.
  2. Measure the Length:

    • For non-countersunk bolts, measure from the bottom of the head to the end of the shank.
    • For countersunk bolts, measure the total length including the head.
  3. Identify the Thread Pitch:

    • Use the thread pitch gauge to measure the distance between threads.
    • Align the gauge with the threads and find the size that matches perfectly.

Measuring Thread Pitch without a Thread Pitch Gauge

  1. Use a Ruler or Caliper:

    • If using a ruler, it's best to use one with fine divisions (millimetres are ideal).
    • If you have a caliper, it can provide a more accurate measurement.
  2. Measure the Distance:

    • Carefully place the ruler or caliper so it lines up with the peaks (crests) of two threads.
    • Measure the distance from the peak of one thread to the peak of the next thread. This distance is the thread pitch.
    • If possible, measure across multiple threads and divide by the number of gaps to get a more accurate average measurement.
  3. Use Online Resources:

    • If you have the bolt diameter and have measured an approximate pitch, you can use online charts or resources to match your measurements to standard thread sizes.

Tips for Accurate Measurement

  • Ensure the bolt is clean for a precise measurement.
  • If using a ruler, consider the smallest divisions for accuracy (usually in millimetres).
  • For damaged bolts, it may be necessary to measure a similar bolt from the same part of the car.

Steel vs Stainless Steel Bolts in Automotive Use

Steel Bolts


  • Strength: Steel bolts are generally stronger, making them ideal for high-stress parts of a car.
  • Cost-Effective: They are less expensive than stainless steel bolts.
  • Variety: Available in various grades to suit different automotive needs.


  • Corrosion: More prone to rust and corrosion, especially in damp environments.
  • Maintenance: May require more frequent replacement or treatment to prevent rust.

Stainless Steel Bolts


  • Corrosion Resistance: Highly resistant to rust, making them ideal for exposed areas or in cars used in coastal areas.
  • Appearance: Maintain their appearance longer due to rust resistance.
  • No Coating Required: Unlike some steel bolts, they don’t need additional coatings.


  • Cost: Generally more expensive than regular steel bolts.
  • Strength: While strong, they may not be as strong as high-grade steel bolts, limiting their use in critical high-stress areas.

Understanding how to accurately measure bolt sizes and knowing the difference between steel and stainless-steel bolts is crucial in automotive work. While each type of bolt has its advantages, the choice between steel and stainless steel often comes down to the specific needs of your vehicle and the environment in which it operates. Always consider the balance between strength, corrosion resistance, and cost when selecting bolts for your car.

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