Armstrong inline circulators are considered 3 piece pumps.
The 3 main parts are the power pack which consists of the motor and motor mount.
The next part is the bearing assembly which consists of the housing, pump shaft and bearings, mechanical seal and may or may not include the impeller.
The last part is the volute. The volute is the part of the pump that connects to the water piping. On a closed system such as a heating or chilling system it can be cast iron since the water is treated for corrosion control. If it is used for domestic hot water it should be made of bronze to prevent the volute from rusting since the water will not contain any chemical treatment.
Armstrong makes 6 different bearing assemblies and several different size impellers. The impeller is the main componet of the pump. The motor and bearing assembly are a means to turn the impeller. The same bearing assembly and or motor can be used on several different size pumps. This makes pump identification very difficult. Motor RPM, horsepower, frame size, bolt dimensions, volute flanges, width between flanges are all things that are used to help identify a pump when the original nomenclature is missing.