This paper investigates the relationships consumers cultivate with mass-market commodities while caring for their authenticity. Drawing on a six-year ethnography of classic Mustang owners communities in France, Switzerland and Belgium, the authors show that, far from being a symbolic value only, or a resource into which people can "invest" in a mechanism of social distinction, authenticity can also appear as a burden that weighs constantly on the relationship between people and things. Indeed, throughout their uses and maintenance, the material integrity of classic Mustangs is of great concern for their owners, who apprehend every breakdown or maintenance intervention as threats that could jeopardize their car's authenticity. For the sake of security, comfort or health, because new regulations come up, or because some original parts are not available anymore, classic Mustangs owners compose with heterogeneous elements, constantly reshaping both their cars and their concerns for authenticity. The authors draw on Hennion's notion of "attachement" to describe the intimate relationship that grows through these arrangements. The notion particularly helps to grasp the ambivalence of the bonds between people and things: while they get more and more attached to their classic Mustang, owners are getting more and more worried. Moreover, throughout this growing relationship and the recurrent material interventions it draws on, the car does not remain passive. It progressively reveals itself, sometimes surprising its owner. Therefore, not only is authenticity "in the making" in this process, the contours of the thing itself evolve, as well as the knowledge of its owner.