A nailing unit (1, 1') for nailing machines, particularly for nailing wood elements on multiple layers, comprising a clamp (40) adapted to keep in the nailing position a nail (C) above an element to be nailed, and a plunger (30) adapted to push the nail (C) in order to make it penetrate into the element to be nailed; an actuator (20, 120) which comprises a pusher element (21, 121) which moves along a vertical direction, the pusher element (21, 121) being integral with the plunger (30) and with a vertical traction rod (51) which is provided with a lower locking element (55), along which traction rod (51) the clamp (40) can slide freely to a lower stroke limit position in which it is prevented from moving further downward by the locking element (55), so that when the clamp (40) is in the lower stroke limit position an upward movement of the traction rod (51) draws upward the clamp (40); the clamp (40) comprises a through vertical tunnel (48) in which the plunger (30) can slide until it reaches the nail (C) when it is held in the nailing position by the clamp (40) and then push it.