The present invention has its object to provide a heat exchanger with plastic tanks type, wherein a seal member is regularly compressed and hooks or tabs of header core crimped in a uniform fashion.Deformations predominantly located in the large sides or sides adjacent to the tank foot of the header, are formed and distributed in the inner or outer wall. The deformations in the periphery, or more particular, on the outside of the header wall, are formed after the brazing process.