Abstract: A coil having a slot portion intended to be arranged in slots in a stator or a rotor of an electrical machine is manufactured by forming a coil part including a bundle of electrically insulated conductors and insulation surrounding the bundle, arranging a portion of the coil part in a mould including at least two mould parts, surrounding the mould with a heat-shrinkable material which is penetrable by an impregnating resin, subjecting the thus composed product to pressing and heating in a tool which together with the mould gives the slot portion its final shape, the mould parts being spaced from each other to provide longitudinal gaps between their confronting longitudinal edges, and the heating of the product causing the heat-shrinkable material to shrink and retain the slot portion in its final shape after removal of the composite product from the tool, thereafter impregnating the insulation with an impregnating resin and transforming the insulation into a solid state.