The field structures of automobile starters and other dynamos prevent a number of problems when any malfunction occurs. Repair of a field structure without removal from the housing or casing of the dynamo is very difficult and removal of the field structure presents serious difficulties. Generally, there is a drive or press fit between an external cylindrical surface of a poly frame assembly or core of the field structure and the internal cylindrical surface of a housing or casing and considerable force is required to effect relative movement especially when the dynamo has been in use for a length of time. In addition, brush and winding components of the dynamo make it very difficult to use conventional types of tools or fixtures, being in positions such as to interfere with direct engagement with the core of the field structure and also being subject to damage from inadvertent engagement thereof as well as from shocks and vibrations which may occur when attempting to remove the field structure. Replacement of a field structure with a reconditioned or new field structure presents similar problems.