The Buick Master Six was an automobile built by Buick from 1925 to 1928. Before then, Buick was using the six-cylinder 242 engine in their high-end cars and a four-cylinder engine in their smaller, less-expensive cars, but for 1925, they dropped the four-cylinder engine and designed a small six, which they called the Standard 6, to replace that end of the market. They coined the name "Master Six" for the high-end cars, now powered by the 255 engine released the year before. Buick named their six-cylinder cars "Buick 6" from 1916 through 1924, and in 1925, divided them into Standard 6 and Master 6. The Master 6 was renamed Series 121 and 129 in 1929 and Series 40 in 1930. All were powered by a Buick Straight-6 engine, with multiple body styles, and starting in 1926 used the newly established GM B platform, which it shared with Oldsmobile. Although Buick produced mostly four-cylinder cars in the teens and early '20s, as it had been doing, the new six-cylinder cars became more common. Buick first made six-cylinder cars in 1914, with a 331-cubic-inch engine, which was originally a racing engine, later used in limited production as the Series 50. The 331 engine was dropped in 1916 and replaced with the much smaller 40 series engine, starting with a 224-cubic-inch in 1916 and 1917, the 242 engine from 1918, increased to 255 in 1924, increased again to 274 for 1926 through 1928. The engine was increased again to 309 for the Series 121 and 129 in 1929 and Series 40 in 1930, after which all six-cylinder engines were suddenly dropped and all models of Buick were equipped with the legendary Straight 8, starting in 1931 and for decades thereafter. The Master Six was Buick's high-end offering, above the Standard Six. It was also manufactured from knock-down kits at GM's short-lived Japanese factory at Osaka Assembly in Osaka, Japan. It was an all-new platform joining the smaller Model 10, and was replaced by the Buick Series 50, and the Buick Series 60. Starting in 1916, when this generation was introduced, different models were known by a designation that changed yearly. It began as the Series D in 1916, followed by E in 1918, H in 1919, and K in 1920. Beginning with 1921 until 1924, it changed to Series 21, denoting the year, to Series 24. Beginning with the letter designations, the Series D had additional numeric designations for the body style, 44, 45, 46, and 47, designating touring car, roadster, sedan, and coupe. The Series E, H, and K continued using the numeric designations for body styles but added additional designations for longer wheelbases and bodywork. The numbered series vehicles added additional body styles called Pullman Saloon, Country Club, and Town Car, adding numeric designations. The year 1924 continued to use the previous numeric designations without any changes. Starting with 1925, the series designation changed back to numeric designations, but this time they signified the wheelbase used. The Master Six was also sold with a smaller vehicle, called the Standard Six, later the Series 115, and Series 116. They were replaced with the Series 40.
