The invention relates to an overcoil balance spring (1) including a hairspring (3) formed in a single silicon part, coaxially with a collet (5), the balance spring including a silicon terminal curve (7) and a silicon elevation device (9) between the outer coil (15) of said hairspring and said terminal curve so as to form a Breguet® overcoil balance spring. According to the invention, the elevation device (9) has a cross-shaped mechanical fastener (17) including at least two opposite arms (12, 12′, 14, 14′), which cooperate with clamping means (23, 25) respectively secured to the terminal curve (7) and the outer coil (15) of said hairspring.