Good thing I read the sub topic, I was going to ask you WHAT TRUCK!!! I'm not 100% sure, but I would be willing to bet the 4300 would be either steel, but more probably, aluminum. If there were any parts that would be fiberglass, it would most likely be the hood. This is usually the standard today, aluminum cabs and sleepers with fiberglass hoods, with the exception of some Peterbilts and Western Stars. If I remember correctly, depending on model, some of the Petes and Stars use aluminum hoods instead of fiberglass.