Expanding exhaust tubing or pipes is a procedure used to enlarge the diameter of exhaust pipes. Not only will this help while working with bent or straight pieces, but if done properly, it will keep you from getting hot spots in the metal as you work.
Step 1 - Bend the Tubes
After you get your angles figured out as to where you want the pipes bent, either bend them yourself or have them bent. During this process avoid crushing the ends. It happens frequently though and when it does you will need to expand them back out.
Step 2 - Expand the End
You can get a cheap expander at discount tool shops for one-time use. Because you are increasing the size of the pipes, you will want to get a connector to run from the old tail pipe to the new size piping you have installed. You can get that at the auto parts store. It will just be one size on the end to fit your tailpipe and one size to fit the tubing expansion. Make sure that when you use it you get the diameter that you need and that it is in a good circle that will fit over the tailpipe you have installed to accommodate the larger size tubing. The expander works by stretching the metal in an even radius. If you apply too much pressure it will cause the edges to crack. You can also take this to an expert and get them to do it for you. However, the charges for such a procedure will cost more than it does to buy the cheap expander that you can use again if you need it.
Step 3 - Use the Expander
When you put the pipe on the expander, you want to either set it to the circumference that you are going need or if there are dies, then insert the correct size die that will fit the adapter. The adapter is one you have placed over the end of the exhaust pipe to accommodate the new size pipe. It is more of a coupler that will keep both ends intact. It has clamps over it to tighten as needed. When you use the expander, do so slowly, take your time. Use it once, then try the pipe to fit it. Take as many times as you need in order to get the correct fit.
Step 4 - Adjust for the Weather
If it is colder out or in, as the case may be, you may need to use some heat to get the desired effect. If you are using a heater in your garage, place the tailpipe against the heater for a few moments to heat it up to make it bend easier. If the metal is cold it is more likely to crack or flare. Always use gloves when doing this as the pipe will conduct the heat down the length of it. If you have a small propane torch, you can set it on low. Just heat it in small amounts in intervals. Try to expand the pipe slowly. Heat a little at a time until you get the results you desire. Again, be careful with the heat.