IN Sheffield, England, at the Davy McKee works, one of four main saddle castings for the Tsing Ma bridge tower is polished and machined to the tolerances required to hold a 1.15-metre diameter main suspension cable. According to engineer Peter Orange, of the Lantau Fixed Crossing office, the Davy McKee works is casting a total of eight saddles required for the Tsing Ma Bridge job. ''Each saddle is made up in three parts. In the case of the tower castings, there is a grillage base, which sits on the top of the tower, an intermediate section and the casting that goes on top,'' said Mr Orange. ''This photograph is only one third of the total saddle,'' he said. The highly specialised saddles support the main suspension cables not only on the tops of the Tsing Yi and Ma Wan towers, but also on the way down to the anchorages.