The Crankshaft of a Car: A Comprehensive Guide

The crankshaft is a critical component in the engine of a car, responsible for converting the reciprocating motion of the pistons into the rotational motion that drives the vehicle. Understanding the key measurable and quantifiable parameters of a crankshaft is essential for engineers and technicians to develop more efficient and reliable engines.

Crankshaft Length and Weight

The length of a crankshaft can vary significantly depending on the engine size and configuration. For a typical four-cylinder engine, the crankshaft length is usually around 200-300mm, while a larger V8 engine can have a crankshaft length of up to 500mm or more.

The weight of a crankshaft is also highly variable, depending on the material, size, and number of journals. A four-cylinder crankshaft typically weighs around 10-15kg, while a V8 crankshaft can weigh as much as 30kg or more.

The material used for the crankshaft is a critical factor in determining its weight and strength. Commonly used materials include forged steel, cast iron, and even lightweight alloys like aluminum or titanium in some high-performance applications. The choice of material depends on factors such as the engine’s power output, the desired weight, and the required durability.

Crankshaft Journals

crankshaft of a car

The journals are the cylindrical surfaces on the crankshaft that support the bearings and provide a smooth surface for the connecting rods to rotate around. The number and size of journals can vary depending on the engine design.

For example, a four-cylinder engine might have four main journals and four connecting rod journals, while a V8 engine could have six main journals and eight connecting rod journals. The size of the journals is typically in the range of 40-60mm in diameter for a four-cylinder engine and 50-70mm for a V8 engine.

The critical measurements for the journals include:

  1. Taper: The difference in diameter between the center and the ends of a journal. Excessive taper can cause the bearings to wear unevenly and lead to engine failure. Typical taper tolerances are around 0.001-0.002mm per mm of journal length.

  2. Out-of-roundness: The difference in diameter between the largest and smallest points on a journal. Excessive out-of-roundness can also cause the bearings to wear unevenly and lead to engine failure. Typical out-of-roundness tolerances are around 0.002-0.003mm.

  3. Runout: The amount of wobble or movement in the crankshaft as it rotates. Excessive runout can cause the bearings to wear unevenly and lead to engine failure. Typical runout tolerances are around 0.002-0.005mm.

Maintaining these journal measurements within the specified tolerances is crucial for ensuring the proper functioning and longevity of the engine.

Crankshaft Measurement Techniques

To measure the critical parameters of a crankshaft, engineers and technicians use a variety of tools and techniques, including:

  1. Dial Indicators: These devices are used to measure the runout and taper of the crankshaft journals with high precision.

  2. Micrometers: Micrometers are used to measure the diameter and out-of-roundness of the crankshaft journals.

  3. Coordinate Measuring Machines (CMMs): CMMs are advanced measurement tools that can create detailed 3D models of the crankshaft and precisely measure its geometry.

  4. Laser Scanning: Laser scanning technology can also be used to create high-resolution 3D models of the crankshaft, allowing for detailed analysis of its geometry and deviations.

  5. Computed Tomography (CT) Scanning: CT scanning is a non-destructive technique that can provide a comprehensive 3D view of the crankshaft’s internal structure and any potential defects or irregularities.

These measurement techniques are essential for ensuring that the crankshaft meets the required specifications and will perform reliably in the engine.

Crankshaft Angular Acceleration Measurement

The angular acceleration of the crankshaft is another critical parameter that can be measured and analyzed to optimize engine performance and emissions. A study published in the Research Gate journal presented a new method to obtain crankshaft angular acceleration using MATLAB.

The researchers processed the original flywheel data through a MATLAB script to calculate the angular acceleration, which is a crucial factor in determining the engine’s performance and emissions characteristics. By understanding the crankshaft’s angular acceleration, engineers can make adjustments to the engine’s design and tuning to improve efficiency and reduce emissions.

Crankshaft Measurement Case Studies

The importance of accurate crankshaft measurement is further highlighted in various case studies and industry resources:

  1. Video Demonstration: A video on YouTube demonstrates the process of measuring the journals of a crankshaft using a micrometer. By taking six measurements at different positions around the journal, technicians can calculate the taper and out-of-roundness of the journal and ensure that it meets the required tolerances.

  2. MED Engineering Article: An article on the MED Engineering website provides a detailed overview of how to measure and evaluate the geometric deviations of crankshafts using various metrology techniques. The article discusses the challenges of measuring crankshafts due to their complex geometry and support conditions, and proposes a formula for accurately assessing crankshaft measurements using metrology fundamentals.

These case studies and resources illustrate the importance of precise crankshaft measurement and the various techniques and tools available to engineers and technicians for ensuring the optimal performance and longevity of the engine.


The crankshaft is a complex and critical component in the engine of a car, and its proper measurement and evaluation are essential for developing more efficient and reliable engines. By understanding the key measurable and quantifiable parameters of crankshafts, such as length, weight, journal characteristics, and angular acceleration, engineers and technicians can make informed decisions and implement effective strategies for improving engine performance and durability.

The use of advanced measurement techniques, including dial indicators, micrometers, CMMs, laser scanning, and CT scanning, provides a comprehensive understanding of the crankshaft’s geometry and deviations, enabling the optimization of engine design and tuning. The case studies and industry resources presented in this guide further highlight the importance of accurate crankshaft measurement and the practical applications of these techniques in the automotive industry.

By mastering the intricacies of crankshaft measurement and analysis, mechanical engineering students and professionals can contribute to the development of more efficient, reliable, and environmentally-friendly engines, ultimately driving the future of the automotive industry.


  1. Calculate engine crankshaft angular acceleration based on original flywheel data
  2. How to Measure Crankshaft Journals Using a Micrometer
  3. Crankshaft Measurement and Evaluation