What are Interchangeable Parts?

Mary McMahon
Mary McMahon

Interchangeable parts are perhaps one of the greatest and least-discussed engineering inventions. These are parts that are designed to fit in any device of the same type, rather than being designed for one specific item, and they revolutionized the world of manufacturing. With their development, the groundwork for mass manufacturing and distribution was laid, and the Industrial Revolution was born.

Eli Whitney used disassembled guns to demonstrate the virtue of interchangeable parts to Congress.
Eli Whitney used disassembled guns to demonstrate the virtue of interchangeable parts to Congress.

Most people take interchangeable parts for granted. When a car breaks down, for example, drivers know that they can order parts and have them installed by a mechanic. These parts are made in a centralized manufacturing facility and stored until they are needed, and they fit in all cars of the same make, model, and year. Often, manufacturers even standardize parts across several models and years to make replacement parts even easier to access.

Interchangeable parts like spark plugs are required to maintain a mechanized society.
Interchangeable parts like spark plugs are required to maintain a mechanized society.

Prior to the late 1700s, such a thing would have been unthinkable. Every manufactured item, from clocks to carriages, was made by hand, with parts uniquely crafted for that particular item. If the item broke, it needed to be brought to a skilled craftsman for repair, and this craftsman would either repair the damaged part or fabricate a new one. This was an expensive and time consuming process, and many handmade goods had unreliable performance records; cannons, for example, could easily misfire or develop other problems in operation.

In the late 1700s, several manufacturers of guns came up with the idea of making guns with interchangeable parts. In one notable demonstration, Eli Whitney brought ten guns into the United States Congress, broke them into their component parts, scrambled the parts, and then put the guns back together. His demonstration proved that it was possible to make parts that were truly interchangeable, and demonstrated their clear benefits: when a gun failed in the field, instead of sending it out for repair, a soldier could quickly replace a missing or damaged part and keep on fighting.

Whitney failed to take the next logical step, which would have been devising equipment to make these parts on a factory line. His guns were fabricated by hand by skilled gunsmiths. John Hall, Simeon North, and Eli Terry did take this step, however, developing equipment that could be used for mass production of products. Today, the concept seems entirely unremarkable and quite logical, but it was nothing short of a miracle for the world of industry.

Interchangable parts laid the groundwork for mass manufacturing and distribution.
Interchangable parts laid the groundwork for mass manufacturing and distribution.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a EasyTechJunkie researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


The illustration of interchangeable "spark plugs" shows injectors.


It really stuck in my mind that guns were the first machines to experiment with interchangeable parts. Of course there was a lot of them and they often broke, but still, there seems to be something uniquely and appropriately American about this line of logic. Imagine all the other machines, that could have benefited from these parts, but guns were the first to reap the benefits.


Someone once told me that great inventions which change the world don't often come from "Eureka" moments of amazing discovery but rather through the slow and steady accumulation of interrelated advances. For instance, computers have changed the world completely in just 50 years, but there was not one moment or discovery which defined the computer age.

The discussion of interchangeable parts reminds me of this. At first it probably seemed minor, nothing more than a convenience. But as other have pointed out, this invention facilitated the rise of manufacturing which built the modern world. It was only once this invention evolved and incorporated itself with other industrial discoveries that its full potential was realized.


I thought the mention of Eli Whitney was funny because he is often considered one of the founding father of modern engineering and logical designs. Of course he is most famous for the cotton gin which industrialized a previously laborious task and led to huge increases in cotton production.

Clearly he saw the benefits of interchangeable parts but his vision was a little short sighted. Really the whole crux of the invention rests on making interchangeable parts on a large industrial scale. If you have craftsmen making each individual part then the benefits are basically lost. Makes me wonder how the cotton gin was produced?


I can't imagine a world before interchangeable parts. Imagine having to get a unique part machined just for you every time any little thing broke. You would have to treat machines like works of art and every little part would be singular and unique.

It makes me admire the craftsmen of the age but also grateful that I didn't live back then. Things would be so frustrating. Imagine how often things break these days and what a nightmare it would be to try to get fixed.


The author is right that this seemingly simple invention does not get the the credit it deserves as one of the hallmarks of modern engineering. Without interchangeable parts the industrial revolution would not have been possible and the world as we know it would look much different. Practically everything you own contain interchangeable parts and was made by machines which contained interchangeable parts. It sounds weird to say, but we should try harder to celebrate this incredible invention.




When were they invented?

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