This is the hard bit! The D-shape rings are made to a tight fit and slotted over each end, tapped on with a cow hide hammer. A 'strut' is placed in position so that when the rings are squeezed together in the jaws of the vice, the desired position is reached when the rings reach the strut.
This is always a tricky operation. If the rings are not right enough and are too baggy, they slip and don't hold their position. The soldering is also very tricky. Due to the large size of the cylinder a great deal of heat is required; the cylinder is placed upon a mesh so that two gas torches may be used simultaneously, one underneath and the other from the top.