A balance system for an assembled engine with an engine axis, comprising: an engine casing having a shaft, an inner case, an outer case; a rotor (14; 16; 18; 21; 22) mounted on the shaft for rotation within the engine casing; and a weight system mounted on the shaft of the rotor (14...22) and having at least one weight positionable relative to the rotor (14...22) around a circumference of the shaft; wherein the inner case has an inner access port, the outer case has an outer access port, and the inner access port, the outer access port and the weight system are positioned on the same radial plane of the shaft, wherein the radial plane is perpendicular to the engine axis, whereby the weight system can be accessed through the outer access port and the inner access port to adjust circumferential position of the at least one weight relative to the shaft, wherein the weight system comprises a rail mounted around at least a portion of the circumference of the shaft, and wherein the at least one weight is mounted to the rail and has a lock structure for locking the weight in position relative to the rail, wherein the lock structure can be locked and unlocked through the inner access port and the outer access port, wherein, when the lock structure is unlocked, the at least one weight can be moved along the rail, and when the lock structure is locked, the at least one weight is fixed in place on the rail, wherein the at least one weight can be accessed from outside the engine casing in a straight line through the outer access port and the inner access port, wherein the inner access port is a borescope port, characterised in that the outer access port is a fuel injector port. an engine casing having a shaft, an inner case, an outer case; a rotor (14; 16; 18; 21; 22) mounted on the shaft for rotation within the engine casing; and a weight system mounted on the shaft of the rotor (14...22) and having at least one weight positionable relative to the rotor (14...22) around a circumference of the shaft; wherein the inner case has an inner access port, the outer case has an outer access port, and the inner access port, the outer access port and the weight system are positioned on the same radial plane of the shaft, wherein the radial plane is perpendicular to the engine axis, whereby the weight system can be accessed through the outer access port and the inner access port to adjust circumferential position of the at least one weight relative to the shaft, wherein the weight system comprises a rail mounted around at least a portion of the circumference of the shaft, and wherein the at least one weight is mounted to the rail and has a lock structure for locking the weight in position relative to the rail, wherein the lock structure can be locked and unlocked through the inner access port and the outer access port, wherein, when the lock structure is unlocked, the at least one weight can be moved along the rail, and when the lock structure is locked, the at least one weight is fixed in place on the rail, wherein the at least one weight can be accessed from outside the engine casing in a straight line through the outer access port and the inner access port, wherein the inner access port is a borescope port, characterised in that the outer access port is a fuel injector port.