A joint assembly for securing flanges of two casings comprising a plurality of bolts, a plurality of collars and a plurality of nuts. Each bolt extends through an aperture in the first flange and an aperture in the second flange, each collar being arranged around a shank of a respective bolt and each nut being secured on a respective one of the bolts. Each collar being arranged between one of the flanges and a nut or a head of a bolt. Each collar comprises a first member and a second member, the second member being arranged at a first end of the first member. The second member being substantially U-shaped in cross-section, the second member having first and second radially inner ends and a radially outer mid portion and an axial space is defined between the first radially inner end and the second radially inner end of the second member.