A plant including: an assembly line along which a plurality of pallets are circulated in a closed loop, divided into a first section, where the pallets are loaded in a plurality of first stations with mechanical parts to be assembled, and into a second section, where the pallets advance along a plurality of second stations; a conveyor arranged at least partially parallel to and above the second section, along which a plurality of balance supports provided with bodies run; a coupling station, arranged immediately upstream of the second section, where the pallets are integrally coupled to the balance supports; and a decoupling station, where the pallets are separated from the balance supports; each pallet having selectively engageable and releasable fastening means for a balance support and elastic means able to receive the balance support resting thereon.