A hairspring collet for a hairspring for interference engagement and an interference fit with the cylindrical outer surface of the staff of a balance wheel for a timepiece movement, said hairspring collet portion comprising a plurality of circumferentially extending elastically deformable interconnected arm portions, the arm portions forming an annulus having a central axis and providing an aperture therebetween, wherein each arm portion including a curved concave engagement portion for engagement with the outer surface of a staff of a balance wheel, wherein each engagement portion has substantially the same radius of curvature as each other and are equally spaced from said central axis at a first distance and wherein said first distance is less than the radius of the staff of the balance wheel; said engagement portions have a radius of curvature such that upon deformation of the arm portions and engagement with the outer surface of said staff the engagement portions substantially conform with the outer surface of said staff and an interference fit is formed therebetween, wherein stress induced from said interference fit is transferred and distributed from along the engagement portions to the arm portions adjacent the engagement portions and distributed therein; and wherein the interference fit of the engagement portions with the staff substantially prevents relative movement between the hairspring collet and the staff of the balance wheel upon application of load from a hairspring in use in a timepiece movement.