How to calculate bore & stroke

Written by shellie braeuner Google
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How to calculate bore & stroke
The volume of the engine is determined by the bore and stroke of the pistons. (Aeronautical piston engine image by Andrew Breeden from

Knowing the bore and stroke of an engine is key to understanding the engine's performance. The bore of the engine is the diameter of the piston hole. The stroke is the distance the piston moves within the hole. Together with the number of cylinders, the bore and stroke give the displacement volume of the engine. Many car makers put the volume of the engine in its product name. For example a 350 Chevy has a volume of 350 cubic inches. The bore and stroke can be determined through the mathematical equation DV= pi/4 x bore x bore x stroke x # of cylinders.

Skill level:

Things you need

  • Wrench
  • Ruler

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  1. 1

    Plug the information into the equation. For a 350 Chevy with eight cylinders, the equation would read: 350 = 3.14/4 x bore x bore x stroke x 8.

  2. 2

    First divide pi by four. 350= .785 x bore x bore x stroke x 8.

  3. 3

    Divide both sides of the equation by 8. This removes the number of cylinders. 43.75= .785 x bore x bore x stroke.

  4. 4

    Divide both sides of the equation by .785. 55.733= bore x bore x stroke.

  5. 5

    Measure the bore. Engines have standard bore sizes for specific models. For example, the 350 Chevy has a 4.12-inch bore. But after market modifications often include boring a wider piston shaft. The most accurate way to pull the engine head and measure the bore. Remove the bolts holding the head to the engine, and measure the bore with a small ruler.

  6. 6

    Plug the numbers into the equation. 55.733= 4.12 x 4.12 x stroke.

  7. 7

    Square the bore measurement. 55.733= 16.97 x stroke.

  8. 8

    Divide both sides of the equation by the squared bore measurement. 3.28= stroke

Tips and warnings

  • For more accurate equations, use all of the decimal places of each step along the way.
  • With the head of engine off, its a good time to change the header gasket.
  • Use great care when removing the engine head and header gasket.

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