This study examines a neglected aspect of research on industrial relations: workplace disputes in which workers are incidentally made idle because of the strike actions of others. The work uses a comprehensive dataset from 1920 to 1970 compiled by the UK Engineering Employers’ Federation (EEF), where membership embraces engineering and metalworking industries, including automotive and aircraft manufacture. During the interwar and wartime period (1920–45), relatively few workers were made idle compared with those directly on strike. In contrast, the post-war period (1946–70) saw a substantial rise in both the likelihood and scale of workers made idle. This was driven largely by sharp increases from the late 1950s with the onset of prolonged and previously unprecedented industrial conflict in manufacturing. Focusing on 1960–70, the study finds that in the later years, when the new Labour Government encouraged mergers in the car industry, the number of EEF workers made idle nearly matched the hundreds of thousands who were striking. Non-pay disputes were most likely to generate incidental idleness within the EEF companies, particularly those involving demarcation disputes, production constraints, working conditions, and redundancy.