Oversquare vs Undersquare Engines – Technical Differences

Harley Davidson motorcycle with engine

Differentiating engines as oversquare, square, and undersquare might sound too technical. However, the basis and the differences are mostly basic and simple to understand.

While there are some engineering technical terms, we will keep the description simple for easier understanding.

How are oversquare and undersquare engines differentiated? If the piston stroke length is larger than the bore diameter, the engine is called an undersquare engine. And if the bore diameter is larger than the stroke length, the engine is called an oversquare engine.

If both the stroke and the bore are of same length, then it is called a square engine.

Before, we dive into the specifics and differences of these engines, it is vital to understand the basics of how an internal combustion (IC) engine works.

Basics of an IC Engine

While Internal Combustion Engine is a big concept in itself, in this post we are just going to touch upon the basic functioning of a 4 stroke IC engine. We won’t dive into 2 stroke engine or the detailed functioning of the IC engine since that would divert the topic of this post.

2 stroke engines are significantly less used in modern motorcycles compared to 4 stroke engines. Although there are still 2 stroke engine motorcycles manufactured currently, they were far more popular earlier as you can see in old vintage bikes.

In a 4 stroke engine, a combustion cycle occurs with 4 strokes of the piston along the cylinder. The piston travels to and fro along the cylinder walls. This rectilinear to and fro motion is converted into rotational motion by the crankshaft. The converted rotational motion is connected and is passed on to the gear box and wheels.

The 4 strokes are typically labelled as –

  1. Intake: Piston goes down to intake the air fuel mixture
  2. Compression: Piston travels up the cylinder to compress the fuel
  3. Combustion: The fuel combusts with the help of spark plug. The energy produced, forces the piston to travel down the cylinder
  4. Exhaust: Piston goes up and pushes the exhaust gases out of the cylinder

These are the 4 strokes associated with each combustion cycle in a 4 stroke engine.

Bore Stroke Ratio

Now, lets define in detail what does a stroke length and bore diameter mean. These two parameters form the basis of differentiation for undersquare and oversquare engines.

The stroke length, or in short – stroke, is the distance travelled by the piston from top to bottom or vice versa along the cylinder lining.

The bore, or cylinder bore, is the diameter of the engine cylinder.

Bore Stroke ratio is defined as the ratio between the bore diameter and the stroke length. The bore divided by stroke gives the bore stroke ratio.

Square, Undersquare, Oversquare Engines

Now, if the bore stroke ratio is exactly 1, that is, bore diameter and stroke length are same, then the engine is called square engine.

If the bore stroke ratio is less than 1, i.e., stroke length is larger than bore diameter, the engine is called undersquare engine.

If the bore stroke ratio is greater than 1, i.e., bore diameter is larger than the stroke length, the engine is called oversquare engine.

Undersquare Engine: Bore/Stroke ratio < 1

Square Engine: Bore/Stroke ratio = 1

Oversquare Engine: Bore/Stroke ratio > 1

Square Engines

Square engines are considered to be the most reliable engines since the wear and tear will be comparatively lower than the other engine types.

This is because the stroke and bore are of same length and as a result, the bearings of the crank will experience much lesser stress. The reduced stress on the bearings will decrease the wear and tear consequently.

Square engines are usually found in conventional motorcycles and petrol engines in the cars. The much higher reliability that square engines offer is desirable in these vehicles since the vehicles themselves offer higher reliability and durability.

Undersquare Engines

Undersquare engines are also called as long stroke engines since their stroke length is larger than the bore diameter.

A key characteristic of undersquare engines (long stroke engines) is their thumping sound. You can hear a ‘duk-duk-duk’ thumping noise coming from motorcycles with undersquare engines.

This thumping noise is generated by the longer strokes produced by the piston. The same thump cannot be heard in oversquare engines because the piston will not be travelling such larger distances unlike the long stroke or undersquare engines.

The main advantage of undersquare or long-stroke engines is that they produce larger torque in the engine. The stroke is longer in these engines. And torque is the cross product of force and perpendicular distance. The longer stroke length in these engines make them to produce higher torque when compared to oversquare engines.

Owing to such high torque producing capabilities of undersquare engines, these are used in heavy duty vehicles like trucks. Almost all diesel engines are undersquare engines since they are used for high torque.

The disadvantage of undersquare engines is that the bore will be smaller. Longer stroke and smaller bore is the hallmark of undersquare engines. However, small bore means small cylinder head. And small cylinder head will translate to smaller valves – both inlet and exhaust.  

As a result, the entry of fuel mixture and the exhaust gases is restricted due to such small valves. That’s why the engines will produce large torque but not high acceleration. The fuel entry is restricted and hence the combustion of fuel is limited. The acceleration capabilities are capped off in undersquare engines.

Oversquare Engines

Oversquare engines are also called as short stroke engines since their stroke length is smaller than the bore diameter.

Short stroke or oversquare engines are considered highly reliable engines. The most reliable are, of course, square engines. After square engines, oversquare ones are the most reliable ones. The wear and tear, the stress on bearings – are significantly lower when compared to undersquare engines.

Since the bore diameter is larger, the fuel intake will be far higher since the valves are designed larger as well. As a result, more fuel is burnt in each combustion cycle and hence, more power and acceleration is generated in each cycle of an oversquare engine.

That’s why, oversquare engines generate more acceleration in the motorcycles. However, the downside of short stroke engines is their low torque due to short strokes and lower mileage.

Oversquare or short stroke engines are used mostly in performance motorcycles for their ability to generate more power and acceleration. However, the compromise comes in terms of mileage.

Another advantage of short stroke engines are that they are compact. The lower stroke gives the provision to design the engine compactly. Add in the reliability and low wear and tear that oversquare engines provide, they make an attractive type for manufacturing performance motorcycles.  

Comparison of Oversquare and Undersquare Engines

Here is a brief comparison of oversquare and undersquare engines across different parameters – Reliability, Torque, Power and Acceleration.

ParametersUndersquare EngineOversquare Engine
Bore Stroke Ratio<1>1
Power & AccelerationLowHigh
Mostly used inHeavy duty vehicles & Diesel enginesPerformance Motorcycles