Simms Motor Units Limited started in 1913 by Frederick Simms at first, to sell and repair components, in particular dynamos and magnetos. In World War I it became the principal supplier of magnetos to the armed forces, mainly from his Simms Magneto Company Limited of New Jersey which he had established in 1910. Another subsidiary was set up in 1915, Standard Insulator Company Limited.

In 1920, following the virtual destruction of the Kilburn works by fire, the company took over a former piano factory in East Finchley, north London. A separate subsidiary to manufacture Simms-Vernier couplings (a method of adjusting the magneto's ignition timing) was set up in Lyons, France. During the 1930s the factory developed in conjunction with Leyland Motors a range of diesel fuel injectors, in particular the Uniflow injection pump of 1937. In World War II the company again became the principal supplier of magnetos for aircraft and tanks, also supplying dynamos, starter motors, lights, pumps, nozzles, spark plugs and coils. Experimentation with compound metals for electrical contacts led to Compound Electro Metals Limited.

The East Finchley factory continued to expand after the war, eventually reaching 300,000 square feet (28,000 m2), and the company took over many other firms. Simms Motor Units was itself taken over by Lucas CAV in 1968. Manufacturing in East Finchley was steadily run down as UK manufacturers lost market share. The factory closed in 1991 to be redeveloped for housing. It is commemorated by Simms Gardens and Lucas Gardens.