GEAR-PUMP DRIVE 6P9057 - Caterpillar

The 28 mm (1.1") orifice in the outlet elbow is a restriction to the coolant flow. This slows the coolant flow rate to permit better venting (discharge of air in solution in the coolant, into the air chamber at the top of the radiator).
Formerly, an elbow without an orifice was used.
In certain heavy load applications, it was possible for the coolant to flow fast enough to prevent good venting. Too much air held in solution in the coolant some times caused water pump cavitation (reduction of water flow). The result was engine overheating.
Adaptable As: The new elbow is a direct replacement for the former elbow as shown in the charts. If an engine has a combustion gas leakage into the cooling system and/or if the radiator cap does not hold pressure, check for high operating temperatures and low cooling system pressure. High operating temperatures and low cooling system pressures are conditions which can cause water pump cavitation. If the operating temperatures are high and the cooling system pressure is low, it is Caterpillar's recommendation to install an elbow with an orifice.