Bore, Stroke and Capacity (Part 1)

KTM LC4 690 Engine

KTM LC4 690 Engine

I’m always interested in reading about the technical specifications about bikes (bigger, faster, better etc), but never knew what all the terms meant – bore, stroke, compression ratio – and more importantly, didn’t know what they meant for the bike and the rider.

After some reading, I’ve got a better idea and want to share my findings with you.  I’ll start with the basics – bore, stroke and engine capacity.

If you imagine the shape that the cylinder creates as it moves from end-to-end within the engine block to be a TUBE, then bore would be the tube circumference while stroke would be the tube height.

Engine capacity, cc, then is the volume of the tube.

Now, if the bike had more than 1 cylinder, just multiply the volume by the number of cylinders for the total capacity.  Let’s take my Duke 125 for example:

Bore (mm) : 58

Stroke (mm): 47.2

Capacity (cm3) = π x 58/2 x 58/2 x 47.2 ÷1000 = 124.8  <– Equation for volume of a cylinder

Since it’s only got a 1 cylinder, that’s the total capacity of the engine.

I tabulated dug out the various bore and stroke combinations of various 125 cc bikes.


You’ll notice from the table that some bikes have bigger bore numbers while others have bigger stroke numbers.  So why is this so?

I THINK (from my initial readings) that bore and stroke affects the engine characteristics, changing it from revvy to torquey.

Let me research more and share my findings with you.



  1. Pingback: Bore, Stroke and Capacity (Part 2) « KTM Mad
  2. Pingback: Bore and Stroke has no effect on Horsepower (Part 1) « KTM Mad

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