Caterpillar typically uses a distinct style fo stack on their heavy machinery, especially the large dozers, excavators, and mining equipment. It's usually a piece of straight pipe that is cut and wedged so that it angles at about 45* for a few inches...usually the bottom length is about half the diameter of the pipe. I've seen "small" 4" dia all the way up to 12" dia sewer pipe. There's a guy on DTR that has an 8" or 10" stack that he's going to install. Looks like he took a culvert and put it in his bed.
If you go to Cat's website and look around at large equipment you will see their stacks.