The object of the invention is a cap to lubricate an inner wall of a perforation going through at least two elements of the structure. The cap comprises a tubular body and at least one lubricant reserve, and before installation in the structure said cap has a largest external diameter larger than an internal diameter (DS) of the perforation, and the cap lubricates the internal surface of the perforation when its largest external diameter is radially compressed by a wall of the perforation. The invention also concerns a fastener comprising this type of cap, and an interference assembly process for the fastener equipped with the cap. The invention is applicable to the assembly of aircraft structures.