# How to Figure Bore-to-Stroke Ratio

by Serena Styles
Large piston aircraft engine image by Andrew Breeden from Fotolia.com

In a piston engine, the "bore-to-stroke ratio" describes the relationship between the cylinder bore and the piston stroke. Bore-to-stroke ratio often aids in engine design, providing insight into the internal combustion rate of a gasoline or diesel engine. For a technical look at your engine's function, you can calculate the ratio with a few simple measurements.

### Step 1

Measure the diameter of the cylinder bore. Find the measurement by measuring straight across the hollow interior of the bore. Write the resulting number down for later use.

### Step 2

Measure the length between the main bearing and the rod bearing of the engine. The main bearing is the bearing on which the engine's crankshaft rotates. The rod bearing is the articulating joint on which the piston rotates. All piston engines have at least one main and rod bearing. Write this number down for later use.

### Step 3

Multiply the length between the main bearing and the rod bearing by two. For example, if your length were 1.5 inches, you would perform the following equation: 1.5 x 2 = 3. This is the piston stroke length.

### Step 4

Divide the diameter of the cylinder bore by the piston stroke length. For example, if your cylinder bore were 4 inches and the piston stroke length were 3 inches, you would perform the following equation: 4/3 = 1.33. This is the bore-to-stroke ratio of your engine.