It is a rectangle core approx. 2 in. thick. I will post Pictures once I pick it up from the radiator shop. I attempted to have it repaired, but seems local shops do not want to mess with it and recommended i just replace it instead. Personally I do not think it is in that bad of condition. I worked well for about 3 months until it developed this leak. I was certain it could be repaired until this shop told me otherwise. I will pick it up today and post some Pictures before I try a second shop. Thanks for your time.