Ford's success inspired imitation and competition, but his primacy remained unchallenged until he lost it in the mid-1920s by refusing to recognize that the Model T had become outmoded. More luxurious and better-styled cars appeared at prices not much higher than that of the Model T, and these were increasingly available to low-income purchasers through a growing used-car market. In Britain, William R. Morris (later Lord Nuffield) undertook to emulate Ford as early as 1912, but he found British engineering firms reluctant to commit themselves to the large-scale manufacture of automotive parts. Morris in fact turned to the United States for his parts, but these early efforts were cut short by World War I. In the 1920s Morris resumed the production of low-priced cars, along with his British competitor Herbert Austin and Andre- Gustave Citroen and Louis Renault in France. British manufacturers had to face the problem of a tax on horsepower, calculated on a formula based on bore and the number of cylinders. The effect was to encourage the design of small engines that had cylinders with narrow bore and long stroke, in contrast to the wide-bore, short-stroke engines favoured elsewhere. This design handicapped the sale of British cars abroad and kept production from growing. It was not until 1934 that Morris Motors finally felt justified in installing a moving assembly line; the Hillman Company had preceded Morris in this by a year or two