DefaultFlattening.cs

public static void Main(string[] args)
{
	Mapper.Initialize(cfg =>
	{
		cfg.CreateMap<PartType, Part>();
	});

	Mapper.AssertConfigurationIsValid();
	
	var result = Mapper.Map<Part>(GetPart()).Dump();
}

public class Part
{
    public string PartName { get; set; }
    public string PartNumber { get; set; }
    public string PartPosition { get; set; }
    public IList<Part> PartReplacedBy = new List<Part>();
}

public class PartType
{
    public string PartNumber { get; set; }
    public PartInformationType Part { get; set; }

}

public class PartInformationType
{
    public string Position { get; set; }
    public string Name { get; set; }
    public IList<PartType> ReplacedBy = new List<PartType>();
}

private static PartType GetPart()
{
   var PartTypeInfo = new PartInformationType
   {
       Name = "SomeName",
       Position = "2",
       ReplacedBy = new List<PartType>
       {
           new PartType
           {
               PartNumber = "22",
               Part = new PartInformationType
               {
                   Name = "SomeSubName"
               }
           },
           new PartType
           {
               PartNumber = "33",
               Part = new PartInformationType
               {
                   Name = "33SubName",
                   Position = "33SubPosition",
                   ReplacedBy = new List<PartType>
                   {
                       new PartType
                       {
                           PartNumber = "444",
                           Part = new PartInformationType
                           {
                               Name = "444SubSubname"
                           }
                       }
                   }
               }
           }

       }
   };
   var part = new PartType
   {
       PartNumber = "1",
       Part = PartTypeInfo
   };
   return part;
}