of the arms 5 the outside diameter of the body 2 is equal to i g u00d7 2 or to s u2212 o u00d7 2 the invention proposes to keep the three values i g and o related to their sum s within respective determined values this stems from various experiments which yielded fairly narrow ranges for the following three proportions the value of o considered is the maximum possible value for the movement of the arm 5 in the hole opener table 1 appended hereto gives for seven hole openers of different sizes ranging in increasing size from size 1 to size 7 characteristic dimensions and the corresponding proportions it is evident from this that preference may be given to the following respective ranges for said proportions for any other identical construction condition proportions outside these ranges gave rise to hole openers which did not perform as well as those whose proportions fell within the above ranges whether in terms of operational reliability life quality of work performance etc for its sliding guidance in the body 2 or in the intermediate support 15 each arm 5 may comprise a portion preferably cylindrical of diameter d the value of which is advantageously at.