Door closing system (1a, 1b) for a door, - wherein the door closing system (1a, 1b) has a first door closing arrangement, a second door closing arrangement and an assembly device with a first assembly plate and with a second assembly plate, - wherein the first door closing arrangement can be removably fixed on the first assembly plate and the second door closing arrangement on the second assembly plate, - wherein at least one engaging pin is arranged on the first assembly plate and at least one engaging sleeve on the second assembly plate, facing one another and aligned in an assembly direction. - wherein the at least one engaging pin and the at least one engaging sleeve can be brought into engagement and thereby the first assembly plate and the second assembly plate can be fixed spaced apart from one another, - wherein the at least one engaging pin has a thread in an end region facing the second assembly plate and a mating thread is formed inside the engaging sleeve, - wherein when the at least one engaging pin and the at least one engaging sleeve are pushed onto each other in the assembly direction, the engaging sleeve expands transversely to the assembly direction so that the at least one engaging pin and the at least one engaging sleeve can be fixed to one another through a screw connection that can be produced and acts in the assembly direction, characterised in that - the at least one engaging pin is arranged in a pin opening of the first assembly plate and through this is aligned to the second assembly plate, and - the at least one engaging sleeve is integrally formed on the second assembly plate. - wherein the door closing system (1a, 1b) has a first door closing arrangement, a second door closing arrangement and an assembly device with a first assembly plate and with a second assembly plate, - wherein the first door closing arrangement can be removably fixed on the first assembly plate and the second door closing arrangement on the second assembly plate, - wherein at least one engaging pin is arranged on the first assembly plate and at least one engaging sleeve on the second assembly plate, facing one another and aligned in an assembly direction. - wherein the at least one engaging pin and the at least one engaging sleeve can be brought into engagement and thereby the first assembly plate and the second assembly plate can be fixed spaced apart from one another, - wherein the at least one engaging pin has a thread in an end region facing the second assembly plate and a mating thread is formed inside the engaging sleeve, - wherein when the at least one engaging pin and the at least one engaging sleeve are pushed onto each other in the assembly direction, the engaging sleeve expands transversely to the assembly direction so that the at least one engaging pin and the at least one engaging sleeve can be fixed to one another through a screw connection that can be produced and acts in the assembly direction, characterised in that - the at least one engaging pin is arranged in a pin opening of the first assembly plate and through this is aligned to the second assembly plate, and - the at least one engaging sleeve is integrally formed on the second assembly plate.