Frank Williams found enough sponsors for 1973 for a new F1 project that materialized with the ISO IR designed by John Clarke, which replaced the FX3. A new chassis was built, which was added to other three IRs obtained from the previous FX3 type. Between 1973 and 1974 the ISO Formula 1 did not get much success, except for two sixth places of Van Lennep and Ganley in 1973 and a fourth and a sixth of Merzario in 1974. In 1975 the cars were entered directly by the Williams team.