longitudinal action exerted by the heat of combustion it should be noted that the external diameter of the portion 45 should be chosen to be slightly less than the diameter of the hole which is capable of being drilled by the combustion of the section 46 and the said diameter will be directly related to the diameter of this section 46 in such a way as to allow this section 45 to penetrate into the hole the increase undergone by the cross section of the bolt has the effect in proportion to the combustion of not only deepening the drilling hole but especially of enlarging the bottom of the said hole fig6 shows diagrammatically the shape assumed by the enlarged bottom of the hole the enlarged bottom is indicated in the drawing by the reference numeral 54 after the lower part 45a of the section 45 has been completely consumed by combustion the upper part 45b of the section 45 filled with ferro silicon granules then comes level with the enlarged bottom of this hole from this time on the oxygen flow is reduced in the bolt so that the materials of the bolt largely melt and the remainder are consumed by combustion the ferro silicon which has a higher density than the molten materials progressively fills the enlarged bottom 54 of the drilling hole the respective lengths of the lower part 45a and upper part 45b of the section 45 are chosen so as to obtain respectively an adequate enlargement of the bottom of the drilling hole.