Available base material:

Pipe 4″ schedule 80. OD 114,3mm (4,5″), ID 87,18mm (3,826″) 

or Pipe 114x10mm. OD 114m, ID 94mm.

OD 98mm Cesaroni casing 98,4-98,5mm.

Fin thickness 5 or 6mm

Milling slots to improve alignment, fixating and welding.