As we continue our discussion on the engine components and operating procedures in this chapter, the only reference to the modifications Mod 2 or Mod 3 will be to point out where a particular component or operating procedure differs in one Mod from the other. Engine Structure The engine structure is a framework for supporting the engine and most of its components and for securing the entire assembly to the ship’s structure. It is composed of a welded steel base made in two longitudinal box sections with the necessary ties, plates, and other structural members. The two sections are bolted together near the center. See figure 3-1. Two pairs of saddles are mounted on the base for supporting the engine cylinder. Vertical stands are welded on these saddles to support the saddles for the accumulator. Between these two stands is a frame of welded channels, angles, and gusset plates to provide trusses and ties for the frame. On the crosshead end of the welded base support plates, webs and gussets support the rails for the crosshead. On this end of the base are welded longitudinal guides for the accumulator assembly. Near the end of this frame and bolted to it is the crosshead stop, which is removed when the crosshead is installed or removed. Constant Runout (CRO) Control Valve Assembly The constant runout control valve (CRO valve) is installed at the fixed sheave end of the Mk 7 arresting engine, as illustrated in figure 3-1. It is designed to stop all aircraft with the same amount of runout regardless of the aircraft’s weight and speed (within the limits specified in current recovery bulletins). The CRO valves of both the Mk 7 Mod 2 and the Mk 7 Mod 3 arresting engines have the same major parts–arresting valve, aircraft weight selector, drive system, and control system. The CRO valve is the heart of the equipment. It controls the flow of fluid from the cylinder of the arresting engine to the accumulator. The other components of the valve are used either to adjust the initial opening of this valve for aircraft of different weight or to activate the valve during the arresting stroke. The CRO Drive System When a landing aircraft engages a deck pendant, or barricade, it withdraws purchase cable from the arresting engine. This action causes the crosshead to move toward the fixed sheave end of the engine. In addition to causing fluid displacement from the engine cylinder, the movement of the crosshead causes the CRO valve drive system (fig. 3-1) to rotate the CRO valve cam. Rotation of this cam forces a plunger down onto a set of levers (fig. 3-4), which in turn forces a valve sleeve and valve stem down to mate with a valve seat to close the valve, shutting off the flow of fluid from the engine cylinder to the engine accumulator, bringing the aircraft to a stop. As stated earlier, the CRO valve is designed to bring all aircraft,    regardless of weight, to a controlled stop while using approximately the same amount of flight deck landing area. This is accomplished by adjusting the allowable opening of the CRO valve, a smaller, more restrictive opening to arrest a heavy aircraft or a large valve opening to arrest a light aircraft. Control Valve Weight Selector The aircraft weight selector makes it possible to adjust the valve for aircraft of different weights by varying the valve opening. See figure 3-4. The size of the initial valve opening is adjusted while the arresting engine is in the BATTERY position. The lead screw receives rotary motion from the motor unit or handwheel and converts it into Iinear motion. This linear motion positions the upper lever and drives the local and remote indicators. In each of the two levers (upper and lower), the distance between the fulcrum and roller is constant. On the upper lever, the distance between the fulcrum and the point of application of force from the cam is variable, its greatest length being twice that of the lower lever. The lever arm ratio of each lever, therefore, is variable between 1:1 and 2:1. When the upper lever is fully extended, the ratio of each lever is 1:1. In this setting the initial opening of the control valve upon engagement of an 3-6