Bore in an engine is the diameter of the cylinder in which the piston moves up and down.

What Is Bore in an Engine?

In a piston (internal combustion) engine, bore or cylinder bore is simply the inside diameter of each cylinder. Together with stroke (how far the piston travels) and the number of cylinders, bore is used to calculate engine displacement (like 1.2L, 2.0L, 5.0L, etc.).

Larger bore generally means a wider cylinder, which allows for bigger valves and can help an engine breathe better at high RPM, while smaller bore with longer stroke often favors torque and efficiency at lower RPM.

Quick Scoop

  • Bore = width of the cylinder (diameter).
  • Stroke = how far the piston travels up and down.
  • Engine size (displacement) comes from bore, stroke, and number of cylinders.
  • A bigger bore (for the same stroke) usually helps high‑RPM power; a smaller bore/longer stroke tends to help low‑RPM torque and fuel efficiency.
  • Boring an engine means machining the cylinders to a slightly larger diameter to increase displacement or restore worn cylinders.

In forum and workshop talk, when people say “bore and stroke,” they’re talking about the basic geometry that gives the engine its character: how big each cylinder is and how far the piston travels.

A Bit More Detail

1. The Exact Definition

  • Bore is the inner diameter of the cylinder, measured straight across.
  • It applies to car and bike engines, but also to things like steam engines or locomotive cylinders.

Because it’s a pure geometric measure, bore is one of the first dimensions engineers decide when designing a new engine family.

2. Bore, Stroke, and Displacement

The basic formula for the displacement of one cylinder is:
displacement per cylinder=π×(bore/2)2×stroke\text{displacement per cylinder}=\pi \times (bore/2)^2\times strokedisplacement per cylinder=π×(bore/2)2×stroke

Multiply that by the number of cylinders to get total engine displacement (for example, a 4‑cylinder 2.0L engine).

3. Oversquare vs Undersquare

You’ll often hear:

  • Oversquare engine : bore > stroke (wide, short cylinder).
  • Undersquare engine : stroke > bore (narrow, deep cylinder).

Broadly speaking (with many exceptions):

  • Oversquare: likes higher RPM, often better peak power.
  • Undersquare: more low‑end and mid‑range torque, often better efficiency and “pull” at lower revs.

4. “Boring” an Engine

When people say an engine is “bored” or “bored and stroked”:

  • Boring = increasing the bore by machining the cylinder walls a bit larger.
  • This slightly increases displacement and can boost power, as long as the block has enough material and is done within safe limits.

Example: a V8 with 400 cubic inches can be bored to get closer to 500 cubic inches in some performance builds.

Simple Example

Imagine a single‑cylinder engine:

  • Bore: 70 mm
  • Stroke: 80 mm

If you increase the bore to 75 mm while keeping stroke the same, the displacement goes up because the piston is sweeping a larger diameter area each stroke. That is why tuners talk about boring as a way to “make the engine bigger.”

Mini FAQ

Is bore the same as engine size (like 2.0L)?
No. Engine size (displacement) uses bore + stroke + number of cylinders together.

Why do spec sheets list bore x stroke (e.g., 86.0 mm x 86.0 mm)?
That tells you the cylinder shape. If the two numbers are equal, the engine is “square.”

Does a bigger bore always mean more power?
Not always—head design, compression, fueling, and tuning matter a lot—but, all else equal, a larger bore can support more airflow and potential power at higher RPM.

TL;DR: Bore in an engine is the cylinder’s internal diameter, a key dimension that, along with stroke and cylinder count, defines engine size and strongly influences how the engine makes its power.

Information gathered from public forums or data available on the internet and portrayed here.