Since Turner first designed the 499cc Speed Twin for Triumph Motorcycles in 1937, the vertical twin has been a big hit with motorcyclists all over the world. The brilliance lay in its compact design and its better than average performance. The Japanese manufacturers were very quick to realise this popular concept, and they have all, at one time or another produced bikes which lean very heavily towards the 'British' look.

In 1954, a company called Meguro built a 500cc OHV twin, called the Senior. It featured a fully-enclosed rear chain, high-rise style handlebars, a two-level seat, telescopic forks and a full width front hub. It sold well, particularly to the police in Japan. By 1959, it had become known as the K1, and was a definite copy of BSA's A7.

However, Meguro ran into financial difficulties and in 1960 they became affiliated to Kawasaki Aircraft.
Kawasaki had begun motorcycle production in 1949, marketing the bikes under the brand name of Meihatsu.
In 1962, the first motorcycles to carry the Kawasaki name began to appear.

In March 1965, the K2 appeared under the Kawasaki name, and in October it was joined by the W1, which was an enlarged 624cc OHV version. The W1 came in a choice of 3 colours, and quickly became the top selling large capacity model in Japan.
It was, however, a disaster in the States. To attempt to appeal to the American public, a number of variations were produced - such as the W2S, introduced in 1967, as a street scrambler with slightly raised compression, and twin carburettors. There was also the similar W1SS in the same year.