Timepiece sub-assembly including a balance spring stud arranged to be fastened to a balance cock, and a balance spring comprising an outer coil and an end plate of larger section than that of the outer coil, this stud including a housing arranged, in a first free state of the sub-assembly, to loosely contain the end plate with tangential mobility with respect to the outer coil, and, in a second immobilized state of the sub-assembly, to immobilize the end plate to which the housing is joined by a layer of adhesive covering a change-of-section shoulder and which forms a mechanical seal for the balance spring in a longitudinal direction of the housing, and the end plate includes an eye communicating with the layer in the second immobilized state.