Gotta make this exhaust part

[Tiger Tim]

I thought I had a handle on how I was going to do it but it’s looking like there’s a good chance I was wrong. How would you guys make this?

 

[slociviccoupe]

Tube bender or tube roller. Weld on a pre made slip fit on the end. Make it all from 321 stainless.

 

[Tiger Tim]

Tube bender or tube roller. Weld on a pre made slip fit on the end. Make it all from 321 stainless.

That was my best uneducated guess but all I’m getting from shops that roll tubing every day is that it can’t be done with thin wall. I don’t doubt you for a second but it sounds like I’ll have to learn enough about it to be able to do it’s myself if need be.

 

[slociviccoupe]

Google cnc mandtel bending. Lots of shops that will do it. Give them your drawing or one that shows center of bend radius. Where ends end up. And they can quote it and bend if you like price. Or find rolled tube and cut and weld. I can get you rmail of company who was very happy to help me with my stainless steel bending

 

[Mcmark]

I just modded a twin Comanche exhaust to fit my Tailwind. Used the pie cut process to make mild directional changes. Have seen the technique used on both motorcycle and hot rod exhaust systems. You Tube is a friend for this.

 

[Chilton]

I have a vague feeling the bends were welded up from 2 stamped parts, have any of the other Gipsy Moth rebuilders made them recently? There must be a trick that "everyoune knows" I would think since there is not likely to be the volume for someone to make them as a business!

 

[pictsidhe]

Thin wall pipe can be bent to a large radius like that without a mandrel bender. Use a pipe or tubing bender. Bend a little, move the tube through 1/2", bend, move, bend, move. Practice on mild steel till you get the hang of it.
I've bent 2" 18g tubing to under a foot radius on a pipe/conduit bender.

 

[Jay Kempf]

Back in the day you would pack the tube with sand to keep it from collapsing on the outside of the bend.

 

[WonderousMountain]

This ^^^ .
It will also work with properly sized wheel.
Plug one end, fill, plug the other end, or as
recommended by professional manifolders.

 

[Tiger Tim]

I have a vague feeling the bends were welded up from 2 stamped parts, have any of the other Gipsy Moth rebuilders made them recently?

You’re right, each bend was originally made of two stampings edge welded together. I had assumed the modern one-off way was to roll the bends into a tube but I’ve also put a bit of thought into how I could hydroform those stampings if that’s what it came to.

 

[pictsidhe]

One way could be to cut two flat pieces of steel. Weld the edges, add a nipple and inflate hydraulically. This is how many two stroke expansion Chambers are made.

 

[lakeracer69]

Back in the day you would pack the tube with sand to keep it from collapsing on the outside of the bend.

This ^

 

[trimtab]

The pie-cut-and weld method works fine. TIG is fast enough for this.

You can also buy pre-bent sections and weld them together. It is pretty economical although radii of curvature are limited.

CNC mandrel benders are hard to find without paying them hundreds in setup costs, and they are justified in charging.

Sand method will collapse or tear...take your pick.

 

[Chilton]

Have you been in contact with anyone like Mike Maniatis who have done Gipsy Moth restorations outside of a company? I am not in touch with any of the DH 60 rebuilders, but I expect there are several easy methods developed by others for this bend. I am glad the long pipe for the Tiger Moth is straight so it only needs a kink on the manifold welded in!

 

[Jay Kempf]

Sand method will collapse or tear...take your pick.

Not based on the drawing in the 1st post. It might not even be needed with a proper bending mandrel. Years ago we had to do some large radome cable bends. Huge coax. Had to design and build a bender that had an inside spool piece and an outside sort of channel both to the right side. Then all you had to do was run a bearing at the right radius off of the capstan center to the outside of the channel piece and run it around. It held the entire diameter tightly and so kept the buckling from starting. We could get much tighter bends without collapse that way.

Another method patented in the heating appliance industry uses a sort of finger or spoon like shape in the bending die inside where the pipe is fed into the machine (a friend of mine was the inventor). This guide touches the outside of the bend from the inside to keep it from collapsing and guides it as it is stretched by the bending force. Hard to explain in words but you can get perfect tight 180 bends in thin wall with these machines. The tubes were used for heat exchanger tubes in furnaces so the tightly packed S bends give lots of path and surface area. These machines were manual at the time but work just like CNC benders today, sorta like a spring forming machine today only with tubing.

Just thought of the other method probably not appropriate for tubing of this size... But using a spring of about the correct inside diameter you insert the tube inside the spring and use a mandrel to bend both the spring and the tube inside. The spring holds the tube round while you bend. Then you have to get the spring back off afterwards. Can be done with the spring inside as well. I would worry that you would collapse the tubing inside or outside the tube and have a hard time removing but maybe a little planning and it could work. Normally this sort of stuff is used to hand form smaller tubes to largeish radii. Maybe just food for thought that can be bastardized in some way.

And the last and most common DIY method is to just buy a kit of exhaust parts from any of several suppliers. A few hundred bucks and you have enough 45 with straight on both ends. 90's with the same, 180 bends, plus straight, etc... to make any exhaust shape. You have to simplify your centerline path on your drawing to use the radius you get in the kit and you start cutting and tack welding until the ends and middle meet the specification and clear all the obstacles. Ebay full of kits like that in various materials and diameters for making headers and full exhaust.

Of course these are all cold forming methods. Get out the torch and another world opens up.

 

[Tiger Tim]

And the last and most common DIY method is to just buy a kit of exhaust parts from any of several suppliers. A few hundred bucks and you have enough 45 with straight on both ends. 90's with the same, 180 bends, plus straight, etc... to make any exhaust shape. You have to simplify your centerline path on your drawing to use the radius you get in the kit and you start cutting and tack welding until the ends and middle meet the specification and clear all the obstacles.

Normally I’d be on board with that, but the end result in this particular case is very visible.

 

[TFF]

I think it’s made from two identical bends, one flipped 180. Most shops deal in industrial. There is off the shelf stuff that’s close like this 1.500" 16ga 15° Mandrel Bend 304 Stainless - (2.250" CLR / 6.000" Legs) if it has to be perfect, it’s just time to pay.

 

[rv7charlie]

I can't read the dimensions on the print, but assuming it's at least 1.5", you should be able to get it done at a well equipped muffler shop. One of my neighbors owns several Midas franchises in the area, and one of their locations has a mandrel bender. I had a pre-bent section of 2.5" ~15" long that I needed another 5 degrees or so added to the bend. Took it to the shop and it was done in less than 5 minutes. IIRC, they could handle up to at least 3", and if they do any heavy truck work, much larger.

Of course if you want it to look 'authentic', you probably need to fab it up manually, with welds all over it.

Charlie

 

[Tiger Tim]

I can't read the dimensions on the print

It’s 2.25” diameter which it turns out is a bit of an odd size and the bend radius is 16” which is an enormous curve by exhaust shop standards, or at least so I’ve found so far.

Originally each curve was made from two stampings edge welded together. That’s great if you need hundreds but nowadays I imagine the best way to get one is to form tube to shape. Of course I reserve the right to be wrong about that.

 

[User27]

Make a wooden jig with the ends at the correct angle and angular displacement from each other. A half-circle cradle can work at each end, allows the pipe to be inserted and removed as the bends are developed. Alternatively pay the money and get a pipe bending company with good CNC equipment to make it.