Yamaha's V4 125cc Racer Won Back-to-Back World Titles in the 1960s

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Yamaha's water-cooled two-stroke V4, the RA31, took Bill Ivy to the 125cc world championship in 1967. Its successor, the RA31A, carried Phil Read to a second consecutive title in 1968. The machine produced 44hp and weighed just 91kg dry, but it became the last V4 in the 125cc class after the FIM restricted the category to two cylinders from 1970.