For the 43, which was a transitional car, Lotus had to fall back on the complex and heavy BRM H16 engine, which was mounted on a pair of chassis. The monocoque of the 43 almost followed the design of the 38 for the USAC series. A first 43 appeared in the 1966 Belgian GP and Jim Clark won the United States GP there. A second unit was prepared in 1967, then the two 43s were sold to private teams who used them in Formula 5000.