A device for assembling and adjusting a balance spring intended to cooperate with a terminal coil of a balance spring whereof the inner end is secured to a balance-staff of a balance, the latter being pivoted in a balance-cock, the device including a base, intended to be assembled to the balance-cock with an adjustable angular orientation, and comprising a substantially cylindrical support surface, with a radius substantially equal to the radius of the terminal coil, and a support sole adjacent to the support surface, said support surface and said support sole defining a service position of the terminal coil, a first arm extending from said base and supporting a locking organ for locking the terminal coil arranged to keep the latter bearing against the support surface, the device being characterized in that it further includes a second arm extending from said base and supporting a sabot able to be arranged at a predefined distance from said support surface to cooperate with the terminal coil, and a control organ movable between first and second configurations and arranged such that it is able to act on the second arm and deform it elastically to move the sabot in reference to said base, said first and second configurations of said control organ being associated, respectively, with a first position of said second arm, in which said sabot is situated at a distance from said support surface equal to said predefined distance, and with a second position of said second arm, in which said sabot is situated at a distance from said support surface greater than the predefined distance. a base, intended to be assembled to the balance-cock with an adjustable angular orientation, and comprising a substantially cylindrical support surface, with a radius substantially equal to the radius of the terminal coil, and a support sole adjacent to the support surface, said support surface and said support sole defining a service position of the terminal coil, a first arm extending from said base and supporting a locking organ for locking the terminal coil arranged to keep the latter bearing against the support surface, the device being characterized in that it further includes a second arm extending from said base and supporting a sabot able to be arranged at a predefined distance from said support surface to cooperate with the terminal coil, and a control organ movable between first and second configurations and arranged such that it is able to act on the second arm and deform it elastically to move the sabot in reference to said base, said first and second configurations of said control organ being associated, respectively, with a first position of said second arm, in which said sabot is situated at a distance from said support surface equal to said predefined distance, and with a second position of said second arm, in which said sabot is situated at a distance from said support surface greater than the predefined distance. the device being characterized in that it further includes a second arm extending from said base and supporting a sabot able to be arranged at a predefined distance from said support surface to cooperate with the terminal coil, and a control organ movable between first and second configurations and arranged such that it is able to act on the second arm and deform it elastically to move the sabot in reference to said base, said first and second configurations of said control organ being associated, respectively, with a first position of said second arm, in which said sabot is situated at a distance from said support surface equal to said predefined distance, and with a second position of said second arm, in which said sabot is situated at a distance from said support surface greater than the predefined distance. a second arm extending from said base and supporting a sabot able to be arranged at a predefined distance from said support surface to cooperate with the terminal coil, and a control organ movable between first and second configurations and arranged such that it is able to act on the second arm and deform it elastically to move the sabot in reference to said base, said first and second configurations of said control organ being associated, respectively, with a first position of said second arm, in which said sabot is situated at a distance from said support surface equal to said predefined distance, and with a second position of said second arm, in which said sabot is situated at a distance from said support surface greater than the predefined distance.