exhaust size?

[Phil Holmen]

What size exhaust is on a stock duramax??

[DarylB]

I believe it's 3.5" mandrel bent SS tubing.

[Kennedy]

The stock tubing is 3.5" aluminized. 2004 and later trucks are 400 grade stainless from the engine to the front of the muffler. This will be obvious in many cases as it will get a coating of fuzzy rust any place that is not aluminized. Any pipe with a catalyst is also aluminized. This follows from the gassers and is intended to fulfil the lenthy emissions warranty. Thing is, the aluminized pipes last plenty long on a diesel, and look better...