Wearable technologies, such as smart glasses allowing assisted or augmented reality (AR) applications, promise to substantially amplify human capabilities in a variety of industrial work contexts. Despite their potential, there is a woeful lack of experimental research that has examined the true performance effects of head-mounted AR displays on actual human-centered operations in industrial settings. To address this research gap, we carried out a series of field experiments with apprentices using smart glasses for a real industrial repair task. We find that even though there are no immediate efficiency gains, the use of smart glasses results in significantly faster repairs over time. The use of smart glasses also has an immediate and positive effect on work quality, but this effect diminishes as apprentices gain more experience. Finally, we find that usability and comfort were no significant problems for the subjects in our experiment.