In 1913 William Brough developed a flat-twin engine in-line with the frame. This 497cc engine had overhead valves, 70mm bore and 64.5mm stroke, and had a magneto fitted above and a 2-speed gearbox below. By the end of 1914 it had replaced all other engines in the Brough range, and using it only 3-models were planned for 1915. The three models were the HS - which was fitted with a two-speed countershaft gear operated by dog clutches, with chain drive to the gearbox, and John Bull rubber belt drive to the rear wheel over an adjustable pulley allowing the top gear range to be varied. No clutch or kick-start was provided. The second model was the HB, which was fitted with a Sturmey-Archer 3-speed hub gearbox and handle starter. The third model was the HTT, which was very similar to the HS but fitted with a specially tuned engine with high-compression, lightweight steel pistons, different camshafts, and special front cylinder lubrication.