A high-capacity radial fit coupling bolt assembly, or high-capacity expanding sleeve bolt assembly, to secure at least two components (7, 107) together, the assembly comprising 1a a shank (1, 101) having a substantially cylindrical central portion (11, 111) intermediate first and second end portions (12, 112, 13, 113); 1b respective inner and outer sleeves (3, 103, 2, 102) surrounding at least a section of the central portion (11, 111) of the shank (1, 101), the inner and outer sleeves (3, 103, 2, 102) having complementary respective outer and inner tapered engagement faces (35, 135, 26, 126); and 1c,i an extrusion ring interposed between a first end of the inner sleeve and a drive piston at or adjacent the first end portion of the shank, the drive piston having at least one axial bore open to the extrusion ring; 1d,i so arranged that, as a tensioning load is increasingly applied to the second end portion (13, 113) of the shank (1, 101), relative axial movement between the inner and outer sleeves (3, 103, 2, 102) can cause the outer sleeve (2, 102) to expand diametrically into engagement with the at least two components (7, 107) up to a preset limit, at which the extrusion ring will at least be partially extruded through the at least one axial bore, thereby allowing the shank to move without displacing the outer sleeve relative to the at least two components; or 1c,ii a compression ring interposed between a first end of the inner sleeve and a drive member at or adjacent the first end portion of the shank; 1d,ii so arranged that, as a tensioning load is increasingly applied to the second end portion (13, 113) of the shank (1, 101), relative axial movement between the inner and outer sleeves (3, 103, 2, 102) can cause the outer sleeve (2, 102) to expand diametrically into engagement with the at least two components (7, 107) up to a preset limit, at which the compression ring can allow the shank to move relative to the inner and outer sleeves (103, 102) and to the at least two components. 1a a shank (1, 101) having a substantially cylindrical central portion (11, 111) intermediate first and second end portions (12, 112, 13, 113); 1b respective inner and outer sleeves (3, 103, 2, 102) surrounding at least a section of the central portion (11, 111) of the shank (1, 101), the inner and outer sleeves (3, 103, 2, 102) having complementary respective outer and inner tapered engagement faces (35, 135, 26, 126); and 1c,i an extrusion ring interposed between a first end of the inner sleeve and a drive piston at or adjacent the first end portion of the shank, the drive piston having at least one axial bore open to the extrusion ring; 1d,i so arranged that, as a tensioning load is increasingly applied to the second end portion (13, 113) of the shank (1, 101), relative axial movement between the inner and outer sleeves (3, 103, 2, 102) can cause the outer sleeve (2, 102) to expand diametrically into engagement with the at least two components (7, 107) up to a preset limit, at which the extrusion ring will at least be partially extruded through the at least one axial bore, thereby allowing the shank to move without displacing the outer sleeve relative to the at least two components; or 1c,ii a compression ring interposed between a first end of the inner sleeve and a drive member at or adjacent the first end portion of the shank; 1d,ii so arranged that, as a tensioning load is increasingly applied to the second end portion (13, 113) of the shank (1, 101), relative axial movement between the inner and outer sleeves (3, 103, 2, 102) can cause the outer sleeve (2, 102) to expand diametrically into engagement with the at least two components (7, 107) up to a preset limit, at which the compression ring can allow the shank to move relative to the inner and outer sleeves (103, 102) and to the at least two components.