Someone posted this to the CB125 discussion group I'm on:
I just thought I would pass along a success story to everyone.
I have a '74 XL125 that has the downpipe exhaust that runs under the engine. Of course, it had multiple dents from rocks, trees and other obstacles encountered by the PO. The expansion chamber has long been rusted off, so basically it is a head pipe that runs to the back of the engine.
One of my friends said you could get the dents out by putting plumbers rubber stoppers in each end, fixing a tire inflation valve into the stopper, and using a compressor to pressurize the air in the headpipe and then using a torch to heat the metal and it would pop right out. He said some blokes used the same process to get rid of dents in gas tanks.
To me that seemed like a lot of work. So the night before last night I filled my headpipe with water, propped it up so that the water was in the section with the dents and let it sit overnight outside. The low was around 20 degrees last night, so this morning it was froze solid and most of the dents were gone. Pushing my luck, I let the pipe thaw in my utility sink during the day and refilled it with water last night and set it outside again. This morning, all of the dents were gone! I REALLY wished I would have taken "before" pictures so I could show you the "after" pictures. Oh well.
Now I have an unobstructed flow head pipe that I can attach a collar to so I can clamp on my supertrapp muffler.
I LOVE IT when a plan comes together!
NOTE: This process might not work on thin wall exhaust tubing and could cause a seam to split, so be forewarned.