The cylinder and ram assembly constitutes the actual engine of the arresting gear. It is located within the engine structure between the movable crosshead and the cylinder outlet elbow on the engines. The cylinder is a machined, forged steel, smooth-bore tube, open on both ends and large enough to provide a working area for the ram and to house the fluid necessary for aircraft arrestments. It is supported within the engine structure by cylinder support saddles. One end of the cylinder is clamped and bolted with four cylinder clamps and Allen bolts to the cylinder outlet elbow. The other end receives the ram. The ram end of the Mod 2 engine cylinder is stepped and counterbored on the inside diameter to accommodate a slipper and cage assembly, a set of V-ring packing, a retainer, and shims. A split wiper and wiper retainer are secured to the end of the cylinder to clean the ram as it moves into the cylinder.   The Mod 3 differs from the Mod 2 in that the packing is installed on the end of the ram instead of at the mouth of the cylinder as on the Mod 2. The ram is a large, hollow steel piston that is moved in and out of the cylinder by the crosshead. It is bored to reduce the weight, although it is not bored completely through. The inner end (the end that fits in the mouth of the cylinder) is solid and provides a working area between the ram and engine fluid during arrestments. As was previously mentioned, the inner end of the ram on the Mod 3 contains a set of V-ring packing to provide a seal for the engine fluid between the cylinder wall and the ram. The inner end of the ram on the Mod 2 and Mod 3 is stepped to accommodate a cage and slippers, which provide a bearing surface for the ram as it slides in and out of the cylinder. The outer (open) end of the hollow steel ram is clamped into a socket on the crosshead by a split flange, which fits into an annular groove near the end of the ram. Crosshead and Fixed Sheave Installation The principle involved in the operation of the crosshead and fixed sheaves is that of a block and tackle. The purchase cables are reeved around the sheaves of the crosshead and fixed sheave assemblies. The crosshead is a three-piece welded structure with two hollow steel shafts clamped between its outer sections and the center section. The crosshead body is clamped to the outer end of the ram by a split flange, which fits into an annular groove around the end of the ram. Figure 3-14 illustrates the crosshead. The crosshead contains two similar banks of sheaves, with nine sheaves in each bank. The sheaves located outboard on the crosshead are 33-inch pitch diameter, and the sheaves located on the inboard shaft are 28-inch pitch diameter. The difference in the pitch diameter of the two banks of sheaves is necessary so that the purchase cable reeved around the outboard (33-inch pitch diameter) sheaves will clear the inboard (28-inch pitch diameter) sheaves. The crosshead sheaves are made of an aluminum alloy casting and have three sections: an inner race, a cage and roller assembly, and an outer race. All the sheaves in the crosshead and the fixed sheave installation rotate on roller bearings with the exception of the 28-inch pitch diameter high-speed sheave that fairleads the purchase cable from the engine to the flight deck. This sheave rotates on a ball thrust bearing. The sheaves are separated by two-section concentric disc spacers. Each spacer has an inner and an outer disc. The inner disc is made of steel and provides lubrication channels from the shaft to each cage roller assembly. The outer disc is made of phenolic and acts as a bearing surface for the outer sheave race; it also retains the grease. Phenolic spacers are bonded to the sheaves on the high-speed side only. The low-speed side has loose phenolic spacers. The sheaves are lubricated through 10 zerk fittings located on the end of each of the two hollow steel shafts (20 zerks total). The crosshead is provided with slipper liners as a bearing surface between the crosshead and the crosshead tracks. There is a total of 16 slipper liners–2 on each side at the top, to prevent side thrust, and 3 on each corner at the bottom, for a bearing surface.    Brass retainers hold the slipper liners in place. The fixed sheave assembly is identical in construction to the crosshead, and all movable parts are interchangeable. The fixed sheave assembly acts as the stationary half of the reeve system and therefore does not require liners. See figure 3-15. 3-18