Apparatus for securing an outer air seal of a turbine rotor stage to a turbine casing comprising: a series of locking key segments for securing the outer air seal assembly of a turbine rotor stage to the turbine casing when the locking key segments are in a locking position; and a split retaining ring for preventing axial movement of said series of locking key segments when the locking key segments are in a locking position, said split retaining ring including: an annular body that is slidably containable within a radially disposed groove in the turbine casing, said annular body having a split therein whereby the body can be flexed radially in an outward direction within said groove, said body being arranged to engage said series of locking key segments to prevent axial movement of said locking key segments; and a series of circumferentially spaced raised lugs located upon the inside wall of said annular body, said lugs having inner surfaces that approximates a circle having a first diameter; the apparatus further comprising: a series of circumferentially spaced seats provided on the locking key segments, said seats having outer surfaces that approximate a circle having a second diameter that is greater than said first diameter when the ring is in an unloaded condition, said lugs being adapted to mount upon said seats whereby the split retaining ring is flexed outwardly in a radial direction when mounted upon said seats. a series of locking key segments for securing the outer air seal assembly of a turbine rotor stage to the turbine casing when the locking key segments are in a locking position; and a split retaining ring for preventing axial movement of said series of locking key segments when the locking key segments are in a locking position, said split retaining ring including: an annular body that is slidably containable within a radially disposed groove in the turbine casing, said annular body having a split therein whereby the body can be flexed radially in an outward direction within said groove, said body being arranged to engage said series of locking key segments to prevent axial movement of said locking key segments; and a series of circumferentially spaced raised lugs located upon the inside wall of said annular body, said lugs having inner surfaces that approximates a circle having a first diameter; an annular body that is slidably containable within a radially disposed groove in the turbine casing, said annular body having a split therein whereby the body can be flexed radially in an outward direction within said groove, said body being arranged to engage said series of locking key segments to prevent axial movement of said locking key segments; and a series of circumferentially spaced raised lugs located upon the inside wall of said annular body, said lugs having inner surfaces that approximates a circle having a first diameter; the apparatus further comprising: a series of circumferentially spaced seats provided on the locking key segments, said seats having outer surfaces that approximate a circle having a second diameter that is greater than said first diameter when the ring is in an unloaded condition, said lugs being adapted to mount upon said seats whereby the split retaining ring is flexed outwardly in a radial direction when mounted upon said seats. a series of circumferentially spaced seats provided on the locking key segments, said seats having outer surfaces that approximate a circle having a second diameter that is greater than said first diameter when the ring is in an unloaded condition, said lugs being adapted to mount upon said seats whereby the split retaining ring is flexed outwardly in a radial direction when mounted upon said seats.