An apparatus for measuring turbine rotor-to-stator clearances and a method for assembling a turbomachine based on the measured clearances are disclosed. In an embodiment, at least one clearance sensor is inserted into a stator of a turbomachine. Using the sensor, tops-on clearance between a rotor blade tip and an inner surface of a stator is measured while an upper stator shell, a rotor, a lower stator shell are assembled together; and a tops-off clearance is measured while the lower stator shell and a rotor are assembled together. A tops-on/tops-off shift, i.e., a difference between the tops-on clearance and the tops-off clearance, is determined. The turbine can be assembled by adjusting a relative position of the rotor and stator to account for the tops-on/tops-off shift.