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      03-14-2013, 08:59 AM   #2
Brigadier General

Drives: 08 E90 M3, 99 E36 M3 Turbo
Join Date: Feb 2011
Location: Concord, NH

iTrader: (3)

The stock pipe may be double walled stainless in places so it looks fatter and the hydro forming may also change the shape in a way that makes it look bigger. As I understand the actual inside diameter is a standard metric size that is in the range of 2.5 inches. Probably 65mm but I have not cut mine up. Someone with test pipes could measure for you. After market midpipes tend to be 2.5 inches though Dinan says its pipe is 2.75.