An assembly/disassembly apparatus (1) for flanged mechanical device comprising: a main body (2) provided with a rear abutment sector (21) having at least a seat (22); a coupling pin (4) housed in partin the seat (22) and extending from the seat (22) in a first direction; a grabbing device (3) coupled to said main body (2); an engagement mechanism (5) provided at least by a first (51) and a second(52) retaining arm, said first (51) and second (52) retaining arms being movable together at least between a first and a second positions, in which,in said first position the first retaining arm (51)faces the rear abutment sector (21) while in said second position the second retaining arm (52) faces the rear abutment sector (21).