Balance wheel-spring assembly of timepiece

A balance wheel-spring system comprising:

• at least one balance wheel which oscillates about a pivot axis via a balance wheel shaft mounted between a bottom plate and a bridge and aligned on the pivot axis;
• and
• at least one spring, one inner spiral of which is fixed to the balance wheel shaft or to a collet mounted integrally with the balance wheel shaft and one outer spiral of which is fixed to a stud integral with the bridge bearing the balance wheel shaft;
• the balance wheel comprising a balance wheel hub, a rim, at least one arm connecting the hub to the rim, a front face of which includes a countersink and a rear face, the at least one arm comprising a housing for receiving and retaining at least one flyweight comprising a head and a rod, and the spring being mounted opposite the front face of the balance wheel,
• wherein the at least one flyweight is mounted on the rear face of the balance wheel, on the at least one arm of the balance wheel,
• wherein the head resting against the rear face of the balance wheel and the rod is flush with a bottom of the countersink, and
• wherein a distance between the bottom of the countersink of the front face of the balance wheel and the spring is at a predetermined interval between 0.05 mm and 1.50 mm.