H-pipes for hemi B-body applications are manufactured of 14 gauge aluminized steel, or optional 14 gauge cold rolled bare steel tubing
for NOS-style applications. 2.50" OD at muffler connection. Please note that the cold rolled steel material does rust internally and externally as the originals did.
Aluminized material is much more corrosion-resistant, which is desired in light of the modern day fuel chemistry.