Triumph Parallel Twin Engine

Edward Turner's 1937 Speed Twin was the most influential motorcycle engine design of the 20th century — a 499cc upright parallel twin that proved more compact, lighter, and smoother than any single-cylinder design of the era. Every subsequent British motorcycle adopted the parallel twin format. Triumph's own lineage runs unbroken: the Speed Twin (1938) became the Thunderbird 6T (1949), then the Tiger 100 (1952), then the Bonneville T120 (1959) — all sharing Turner's fundamental paired-cylinder architecture with 360° firing interval.