Thin wall thickness of the material means it kinks or goes oval very easily. You can try packing it with sand and plugging up the ends to bend it ( putting a spring inside is a plumbers method for bending copper but would be unlikely to work on metal pipe, you'l still end up with a kinked pipe, just with a kinked spring stuck inside! there are other methods) and a mandrel bender is going to do better than one of the £100 machine mart style benders. Somewhere I have a custom bike magazine detailing how to build a diy mandrel bender for exhaust bending from a scrap brake hub and drum for this kind of job. or buy lots of ready made bends but I found that was looking likely to add up to quite a bit of cash just to get 8 bends for the downpipes. Then there are lots of welds. I'll report back when I have time to weld up my current batch of repaired exhausts using a few sections of stainless like you show to repair my cb250s pipes that have rotted their balance pipe out on two sets of pipes now, ditto a ca125. It worked out very cheap as a way to get pipe to sleeve over the rotted area and re make a balance pipe, so far I am very inpressed with the look of the pipe, quality of the swage to sleeve it over itself and the service of the supplier but I have yet to try to get bends made. I reacon a £100 bending machine such as I have would be fine to tweak a bend by say 15° or so in 1 mm pipe, but from expirience using mine, these benders sub 2mm pipe just collapses its wall too easily and ends up with kinks and horrid flats at about 45° when you need about 95° bends for the headers. For my stationary engines I have resorted to cutting and welding pipe at 45° in a few places to get an approximate of the bend I want, and also just cutting up mild steel exhausts for cars. It works out cheap enough and works for a stationary engine, but getting those 8 bends with 4 that match top, and bottom could prove a pain. Might be worth asking about a reduction in price from the supplier of ready made pipes and bends to do say 2 sets of exhausts worth of 90° bends, then alter the ammount of bend a bit at home and weld up.
