The present disclosure relates to a method to apply an insulation to a retaining ring for end windings of an electric machine and to a fillable bag to apply an insulation to a retaining ring. Disclosed is a method to apply an insulation to a retaining ring for end windings of an electric machine, with the steps of providing a retaining ring housing, attaching an insulation at the inside of the retaining ring housing, and inflating a bag inside the retaining ring housing to apply pressure to the insulation. Also disclosed is a fillable bag to apply a pressure to an electric insulation inside a retaining ring for end windings of an electric machine.