Balance spring boot (1) to be driven onto an index (10) for adjusting a balance spring (20) pivoting relative to an axis (D) and including two pins (2, 3), parallel to each other in one direction (D1) and arranged for limiting the radial beat of a balance spring (20) inserted between said pins, and including a means of resting on said index (10) arranged, when said balance spring boot (1) is assembled on said index (10), for adjusting said direction (D1) parallel to said pivot axis (D).Said spring boot (1) includes in a parallel direction to said direction (D1) a sight hole (4) clear so as to allow visualisation by an operator or instrument of said pins (2, 3) and the portion of a balance spring (20) inserted between them at a distance from and/or in contact with one and/or the other of said pins (2, 3).