The Aurora Automatic Machinery Company of Aurora, Illinois entered the motorcycle field in 1903 as an engine and parts supplier, but quickly grew into a formidable industry leader. Aurora was the exclusive engine supplier for Indian Motorcycles until 1907 and was also among the first of America’s motorcycle companies to hire motorcycle racers to drive its products in competition. To this end, Thor motorcycles enjoyed some of the widest brand recognition and greatest domestic racing successes of the period.

By 1908, Aurora and its Thor motorcycles had developed to the point that most of their components were, rather uncommonly, designed and developed entirely in-house. The latest Thor motorcycles were reliable with a clean design and relatively advanced technology for the time. The all-new Thor IV, an example of which is offered here, expanded on those virtues when it debuted in 1910. It was to be the company’s final and most highly developed single-cylinder offering. In 1912, Aurora followed the market by shifting its entire motorcycle catalogue to twin-cylinder “V” engines.

Available with either belt drive or chain drive, as featured on this example, the Thor IV’s single-cylinder, four-horsepower engine featured an automatic intake valve, a proprietary Thor carburetor design, and two oiling systems. A new loop frame provided a lower saddle, while Aurora also used the Thor IV model to introduce its Yielding and Compensating sprocket, a novel precursor to an engine-mount design that isolated engine vibration from the bike’s frame.

This 1910 Thor IV Model K features the company’s unique pneumatic front fork and roughrider-style handlebars, as well as a front mudguard, total-loss battery ignition, and classic retro-white tires. Since entering the consignor’s collection some 30 years ago, this previously restored Thor IV has remained on static display within a climate-controlled gallery.

As one of few surviving examples of the Thor’s most highly refined single-cylinder model, this is a tremendous find that is sure to intrigue collectors of early American motorcycles.