The monkey wrench, known as gas grips in the UK, is an adjustable wrench, a later American development of eighteenth-century English coach wrenches. It was popular in the nineteenth and early twentieth centuries but is now used only for heavier tasks, having been mostly replaced by the lighter and sleeker shifting adjustable spanner. The term monkey wrench is also used colloquially to refer to the pipe wrench, owing to their broadly similar shapes.
These are also known as a Ford wrench owing to this type of wrench being included in the tool kit supplied with every Ford Model A. They are still used by aircraft technicians, chiefly when large but low torque fasteners are involved.
It is currently called an "Adjustable Auto Wrench".
The World English Dictionary gives a nautical definition for monkey, as a modifier "denoting a small light structure or piece of equipment contrived to suit an immediate purpose: a monkey foresail ; a monkey bridge."
Adjustable coach wrenches for the odd-sized nuts of wagon wheels were manufactured in England and exported to North America in the late eighteenth and early nineteenth centuries. They were set either by sliding a wedge, or later by twisting the handle, which turned a screw, narrowing or widening the jaws. In 1840, Worcester, Massachusetts knife manufacturer Loring Coes invented a screw-based coach wrench design in which the jaw width was set with a spinning ring fixed under the sliding lower jaw, above the handle. This was patented in 1841 and the tools were advertised and sold in the United States as monkey wrenches, a term which was already in use for the English handle-set coach wrenches. For the next eighty-seven years a very wide and popular range of monkey wrenches was manufactured by Coes family partnerships, licensees and companies, which filed further wrench patents throughout the nineteenth century. Some Coes wrenches could be bought with wooden knife handles, harking back to the company's early knife making business. In 1909 the Coes Wrench Company advertised a six-foot-long "key" wrench, shaped like a monkey wrench, for use on railroads. The Coes wrench designs were acquired by longtime toolmaker Bemis & Call of Springfield, Massachusetts in 1928. After 1939 its successor companies manufactured monkey wrenches from Coes designs until the mid-1960s, yielding a production run of over 120 years.
Monkey wrenches are still manufactured and are used for some heavy tasks, but they have otherwise been mostly replaced by the shifting adjustable wrench, which is much lighter and has a smaller head, allowing it to fit more easily into tight spaces.
The following story can be found in sundry publications from the late nineteenth and early twentieth centuries:
That handy tool, the "monkey-wrench", is not so named because it is a handy thing to monkey with, or for any kindred reason. "Monkey" is not its name at all, Charles Moncky, the inventor of it, sold his patent for $5000, and invested the money in a house in Williamsburg, Kings County, where he now lives.
However, this was refuted by historical and patent research in the late nineteenth century.
In 2015 it was circulated on social media via viral picture-sharing that African-American boxer Jack Johnson invented and patented the wrench and the term "monkey" was applied to it as a derogatory name for black people. In fact, Johnson did receive a patent for a wrench, but this was only improvements on an existing wrench, which was not related to the monkey wrench, which had been so-named almost a century prior. He patented his improvements on April 18, 1922, as US Patent 1,413,121.