How To: make an exhaust pipe
There are several ways to make an exhaust pipe. I decided to bend a piece of tubing instead of buying some bent pieces and some straight pieces and welding them together. The first attempts didn’t work very well, but I’m glad to describe what I learned during my mistakes, especially if it helps others to successfully bend some tubing.
A friend had sent me a sketch of what the pipe should look like:
This image shows a purchased piece of bent tubing on the left, and a bent exhaust pipe on the right. The one on the left was about $7, but it is too short, so a welder would typically just attach a straight piece to the bottom. But the radius of curvature is way too tight on the purchased tube. I checked with several of the vendors who sell tubes for do-it-yourself exhausts and headers. They use very good mandrel benders, but the radius of curvature is pretty small due to their tooling. It didn’t look right for mine, so I burned $7 on that experiment and went about making one from scratch.
All the old books describe the tips for bending thin wall tubing. It doesn’t have the stiffness of thicker walled pipe, and it will crush easily. We saw this when making up the new chainstays for the frame of the Veloce. One tip is to fill the tubing with dry sand, and then to make a wooden bung to close off the end of the tube and also compress the sand. That was done easily enough with some sand from the sandblasting cabinet and a bit of wooden dowel that was turned down in the lathe. The standard CAUTION from all the books is to make sure that the sand is dry. It seems that wet sand will provide a stronger filler for the tubing. But when we apply the oxy/acetylene torch to the tubing, the sand gets very hot. Hot enough for the water to boil into steam, which expands. That expansion can possibly crack the tubing, or more likely, pop the wooden bung from the end of the tube. Then the hot steam and sand can burn you. So, that is the caution. I used dry sand, it worked ok.
One thing that I have learned in science and engineering is that it is important to have a measurable goal when working on experiments and prototypes. For this project, that meant a drawing of the pipe at 1:1 scale from the CAD layout, and a bit of wire rod that was bent to match the radius of the tube. I could then measure the workpiece to know where it needed more or less bending. You can see that my paper drawings became burned a bit when I put the hot pipe next to it for a visual check! They also got stepped on a few times, sorry for the shoe marks.
Another tip is to mark the workpiece with a soapstone, showing where the bend starts and finishes. This is where the bend straightens out and becomes tangent to the straight sections. On the drawings, these points are marked with blue ticks. Leave a little bit of extra tubing length on the ends of the straight portions, it might come in handy when you need to clamp the tube into the vice or other fixtures.
That’s Andrew Repton from Australia. He was in town one afternoon for work, and stopped by to say hello. Within a minute or three, he had the torch fired up and was showing me his method for bending tubing. I had been heating about 5” of the length of tubing, then bending it around a forming tool. It didn’t work very well, as it was difficult to get that much of the tube to be hot enough with just one torch. Andrew’s method worked much better. He heated about 1-2” of the tube, then bent it slightly using his right hand in the photo above. Then he heated the next 1-2” section and bent it slightly. This repeated a few times until the entire length of the bent section had been heated and curved a bit. Then he started again at the first 1-2” section, and bent a little more. With that explanation, it was time for him to get to his dinner, and he gave me enough ammunition to do the exhaust pipe myself. I went over each inch or so a few times with heat and light bends, until the final curvature was correct.
The tube should not get bright red. A dull red is all that is needed for bending. And if you bend too far, simply bend it back. But only heat the portion of the tube that you want to bend. Re-packing the sand every so often is a good idea too, to keep it tight.
Once the bending was done, the tube needed a flange to fit the cylinder head. The old books had a solution again; to make a forming tool. I used the lathe, but it could be done with a drill and files or countersinks. The important part is that the inner edge of the hole in the tool should be rounded over. The through hole needs to be just large enough to allow the tool to slide onto the tube and back off, around the bend. If you aren’t careful there, the tool could become stuck.
With the tool and the tube supported in the vice, I used a ball pein hammer to form the flanged lip on the pipe. Don’t bother to make the flange in too large a diameter, as it has to fit inside the exhaust pipe nut. And larger flanges are harder to form than small ones that displace less metal. This one ended up just a touch over the required diameter, then I tuned it up with a flap wheel.
Here is the new pipe with the old exhaust pipe nut. Click here to see the new exhaust pipe nut. This is after cleaning the pipe, but before polishing and nickel plating.
Placed onto the motor to check the fit. This photo was months ago, before painting and plating the parts and before the Veloce script chainwheel was made and installed onto the pedal cranks, and the rear wheel still didn’t have a full sell of spokes.
Here is the pipe, the three piece muffler/silencer and the magneto platform after they returned from the first nickel plater. That shop understood industrial nickel plating onto new parts, but didn’t really get the whole process required for old parts; re. the copper buildup, polish, then nickel. I’ve since found a better plating shop and sent most of the parts to them for a second trip through the works. It only costs twice as much…