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How To: make an exhaust pipe

October 26, 2012

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:

veteran ex pipe

 

exhaust pipe 1.5

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.

 

exhaust pipe 01All 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.

   exhaust pipe 3

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.

 

exhaust pipe 2

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.

 

exhaust pipe 4

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.

 

exhaust pipe 5

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.

   exhaust pipe 6

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.

 

exhaust pipe 7

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…

exhaust pipe 8

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27 Comments leave one →
  1. October 26, 2012 8:57 pm

    You’re getting pretty darn handy there, Pete! And by the way, the rear axle you made for my ’33 KTT Velo is still going strong…12 years later. Good work, matey.

  2. Alan Comfort permalink
    October 26, 2012 9:53 pm

    thanks for sharing this technique. do you heat the steel to the same dull red when forming the flange?
    alan in vancouver

    • October 27, 2012 10:30 am

      you know Alan, I cannot remember! I think that I tried it cold first. If that didn’t work well, then I probably heated it up to a dull red and hit it some more. That thin wall tubing isn’t too difficult to form with some choice hits from a medium-sized hammer.

  3. October 26, 2012 9:57 pm

    Pete, I had to make a flare for the front pipe on my 1914 Excelsior twin. I cut short cuts into the end of the tube and bent over 1/8″ square tabs 90 degrees to make the flange from a ring of tabs. Then I brazed up the cuts. Easy. It works ok but the angle of the flange of the pipe is off a few degrees so I’m going to hammer it some and goop it up and add a gasket, original anyway, too to get a seal. As far as the plating goes, how polished were the originals? I’m probably not plating any of the reproduction parts on my bike because they would stand out way too much. Older bikes’ nickle often just looks like bare iron anyway with a lot of the nickle worn or flaked off. If the buyer of the bike wants to he/she can have at the finish. Bikes are only original once and I have some nice surviving paint and graphics too nice to destroy. Thanks, Paul Venne

    • October 27, 2012 10:19 am

      Hi Paul. One solution is to have the new parts bead blasted, then to nickle plate them. It will be a uniform dull silver. It won’t match the original stuff that has flaked off, but it won’t look too terribly new either.

  4. October 26, 2012 10:00 pm

    Pete, The can part of the muffler wouldn’t have been stove black? Do you know the original finishes for teens Excelsior’s mufflers? Thanks, Paul

    • October 27, 2012 10:17 am

      Hi Paul. I think that all the X mufflers were done in black paint, not plated. good luck. Pete

    • October 27, 2012 10:28 am

      For British bikes of that era, they used nickel plating on the exhaust. I have an old one that was made from steel, and some of the plating is still showing on it. But it has been rusted and dented a bit too much for use on this bike. Excelsior used black paint though. I have some good paint for cast iron stoves. I think it was made by Rutland. Full of stinky chemicals, but it works really well for exhausts and iron cylinders, heads, etc.

  5. jon szalay permalink
    October 26, 2012 10:33 pm

    pete , one thing i have found is to use a steel plug and really pack the dry sand in tightly , hammer pack in the steel plug then weld the plug in with a few good tack welds,, nice work pete!!

    • October 27, 2012 10:26 am

      I thought about that, but I wanted to keep tightening the plug. So I made it extra long, and just gave it a hit with a hammer every few minutes to push it into the pipe a little bit further, if it would go.

  6. Doug Lyon permalink
    October 27, 2012 12:36 am

    Another great job – this is really coming on now! I did my Precision pipe cold and full of sand round a flanged hardwood former, as I can’t raise enough heat. Plenty of tweaking required afterwards as the behaviour of the tube seam is a little unpredictable – it still hasn’t quite made it to the platers yet, and after a year’s successful running I don’t think they’ll want to put it in their bath……Keep up the good work!

    • October 27, 2012 10:25 am

      Many platers will take a used exhaust pipe, as long as they can fit their cleaning brushes through the pipe. They won’t do fishtail mufflers though, as they need to get the brushes all the way through to clean out the old carbon.

  7. October 27, 2012 2:20 am

    Completely off topic :-

    Do you perchance have a template for putting a T120R motor in a wideline featherbed? ; as you can guess, I’m getting the parts together to build a Triton this winter :-)

    • October 27, 2012 10:16 am

      I haven’t played around with a Triton before. But I bet that Unity Equipe could get you some information. Good luck, send us some photos of your project.

  8. David Gilliland permalink
    October 27, 2012 3:33 am

    Hi, Your generous points about what not to do are well made and this whole process description is much appreciated. Thank you. David Gilliland

  9. Bruce Williams permalink
    October 27, 2012 5:46 am

    I just made the high pipe exhaust for my R-75 BMW ISDT replica. The system is available for $3700.00 with a steel mufffler. I made it from two sets of used BMW headpipes, a $15.00 U bend from Summit Racing, and a $20.00 used oval alloy silencer. All told, 17 peices cut and welded together. I fit them and just tacked the parts together with a mini mig. Had a pal with a tig do the real welding, gave him $50 in used parts for his pay ( he didn’t want anything). It looks awesome. It is amazing what can be done in an old garage. I am still making the clutch parts for the 09 NSU, thanks Pete

  10. Kris Thompson permalink
    October 27, 2012 2:19 pm

    Very helpful Pete. Like you I have tried this before heating a larger area and found that didn’t work very well either. I’ll give this a try. THANKS A BUNCH!!!

  11. Derek Reay permalink
    October 27, 2012 4:21 pm

    Hi Pete, Another great project, very imformative and an interesting read. The nice photos accompaning the write up add to this intuitive process. Can I please ask if you will be adding any type of Baffle plates to the exhaust, or is the plain open cylinder silencer quieten the machine sufficiently?
    Thanks,
    Derek
    U.K.

    • October 27, 2012 5:37 pm

      There is an external plate on the end of the silencer can with four holes in it. 1/2″ diameter if I recall. That plate can be rotated on a pivot via my left foot, to open or close the holes. But that is more of a cutout than a baffle, as the little holes in the bottom of the cylinder are always open to atmosphere. So I guess the answer is no, no baffles. This is the same design as the Premier and early Triumphs, etc. They don’t seem to make much of a bark with the 4.5:1 compression and mild cam openings. My old 1914 Rudge race bike had only an open pipe, and it was quieter than our 1930’s bikes with fishtail silencers.

  12. bill c. permalink
    October 27, 2012 7:15 pm

    Thanks, great info. FWIW even a MAP torch puts out enough heat for smaller tubing using sand bending, I made a rack that way out of 1/2″ 16ga and it came out OK…have also seen a few videos on YouTube where charcoal was used to heat up bigger tubing, this one is especially interesting as it is also on making an exhaust pipe for a vintage bike

    • October 27, 2012 7:51 pm

      That was a nice setup in the video. Pretty clever using the charcoal to heat the tubing.

  13. oski permalink
    October 28, 2012 12:07 am

    Perchance, who’s the hotshot plating shop you use what knows how to do proper nickel plating?

  14. Sally & Andrew Repton permalink
    October 28, 2012 5:57 am

    Hi Pete

    That’s one way to see that I’m losing my hair . (Your pic of the tube bending .)

    The Yale has progressed heaps since our visit . We have the right colour paint and I’m into the final assembly before braking it down to paint plate and engine rebuild .

    Regards Andrew

  15. October 29, 2012 8:53 am

    I would like to add three suggestions to the pipe theme. First — when filling the pipe with sand it is important to sag it as much as possible and that can only be done by clapping the pipe with a piece of wood on the side of the tube, not from the end of it. It is of course placed vertical while doing it. Second — experienced platers avoid using copper plating on the first 8 or so inches of the exhaust pipe at the end where it is joined to the cylinder. The reason is — copper plating can lift and form bubbles under nickel and/or chrome because of the heat! Finally — an ideal template for transfering or finding out the right form of the exhaust pipe bends if you do not have an original, can be made of a soft round foam polyethylene rod of the right diametre (used for filling gaps between brick walls in building) and a soft copper wire punched lengthwise through the middle of it. It keeps the shape you bend it and follows the set radius.
    My twopennyworth of info…

    Juris

  16. Maina permalink
    March 16, 2013 12:38 am

    Love the handy work and easy to follow visual instructions, it actually does look like an exhaust pipe purchased from the shop.Is it possible to fit in a noise reduction plug incase i need to shield that loud sound.I think it would be applicable to have it possible to fit such a useful gadget in your Motorcycle Exhaust system.

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