A method is disclosed for making an isochronous balance-wheel-and-spring assembly for a horological movement in which one spring out of a series of springs is paired with a balance wheel. The spring has characteristics that vary by comparison with other springs of the series, and the spring is designed to be mounted on the staff of the balance wheel via a collet. One collet out of a set of collets of different sizes is selected on the basis of the spring's characteristics. The collets of different sizes have connection points for attachment of the spring that are located (after assembly) at different distances from the center of the balance staff, the choice of the collet of the most appropriate size facilitating the adjustment of the balance-wheel-and-spring assembly.