SWITCH AS-DISCONNECT 7H7290 - Caterpillar

3400 Series Engines (Direct Injection)Description of Change: The connecting rods and pistons now used in 3400 Series Engines with direct injection are of a different design. As shown in the first illustration, the sides of the new connecting rod and the inner part of the new piston in the area of the piston pin bore are tapered. As shown in the second illustration, the sides of the former connecting rod and the inner parts of the former piston were straight. Also, the top edge of the crater is changed in the new piston. The new piston is stronger than the former piston for two reasons. More of the piston pin is in the bore of the new piston. This causes a distribution of the stress in the piston pin bore over a larger area. Second, the material in the new piston is stronger than the material in the former piston.
The design of the crater in the new pistons gives better combustion of the fuel.
Illustration 1.
Illustration 2.Adaptable To: The new parts are adaptable to earlier engines when used together but a complete set of new pistons and connecting rods must be installed. On 3406 Truck Engines with direct injection, the fuel ratio control must be set at +.035" (0.89 mm) when the new pistons are installed. Since the new pistons and connecting rods do not have the same weight as the former pistons and connecting rods, the new and former parts must not be mixed in an engine. If the new and former parts are used together in an engine, the result is added stress and vibration in the engine.