Double-walled exhaust pipes: how made?

[Owen L.]

How were the original double-walled pipes made – was it merely sliding/pressing one inside the other and then bending to shape?

Is anyone reproducing these today?

[Mark F.]

Re: Double-walled exhaust pipes: how made?

https://bendtooling.com/encyclopedia/double-wall-tubing/

Also called laminated tubing, this type of tubing consists of two strips of coiled steel, on atop the other, rolled together in a tube mill producing, in effect, one tube inside another. Double-wall tubing is most commonly seen in the automotive industry for exhaust systems. The tooling and set-up for rotary-draw bending of double-wall tubing is the same as regular tubing except for the specification of the mandrel assembly. Because, obviously, material cannot flow from one wall to the other as it becomes plastic at the point of bend, the total thickness of the double-wall is not relevant for specifying how much mandrel support the tube requires. Instead, the thickness of the interior wall should determine the number of balls, nose and ball diameters, and the nose placement of the mandrel assembly

https://www.youtube.com/watch?v=GxfoIJoOBJs

I don't know if anyone is reproducing them or not...

[Owen L.]

Re: Double-walled exhaust pipes: how made?

Thanks for those links. Interesting - never considered the inner and outer were formed at the same time but makes perfect sense.

Now, about those inside bend wrinkles and squashed areas in the factory pipes...

[James G.]

Re: Double-walled exhaust pipes: how made?

HULT pipe bending machines have "Controlled crimp" dies to make the wrinkles.

I am attempting to have doublewall or laminated 2-1/2" 68-73 4spd exhaust made - I have a friend who manages a large fabrication shop and they are expecting delivery of a large CNC pipe bender he also said he was pretty sure he could get laminated pipe - his machine will bend up 6 in so it's more than adequate -machine install is still on schedule for this spring -
The manufacturer will be on site for training and teaching etc for a number of days. He said he could get the rep to use my NOS pipes and then the CNC program would be in the memory. I am not sure they would be controlled crimp, however they will have ZERO neck downs in the bends, they use internal "oblong" balls inside the tube to keep the internal diameter the full 2.5"

[Gary B.]

Re: Double-walled exhaust pipes: how made?

A very good video on you tube shows pipe bending with mandrels, or shoes.
Watch... https://www.youtube.com/watch?v=4LMKb0TGAgo

It's uses a HUTH Pipe Bender.

Gary B

[James G.]

Re: Double-walled exhaust pipes: how made?

Thanks for Company name correction Gary...

[Owen L.]

Re: Double-walled exhaust pipes: how made?

HULT pipe bending machines have "Controlled crimp" dies to make the wrinkles.

I am attempting to have doublewall or laminated 2-1/2" 68-73 4spd exhaust made - I have a friend who manages a large fabrication shop and they are expecting delivery of a large CNC pipe bender he also said he was pretty sure he could get laminated pipe - his machine will bend up 6 in so it's more than adequate -machine install is still on schedule for this spring -
The manufacturer will be on site for training and teaching etc for a number of days. He said he could get the rep to use my NOS pipes and then the CNC program would be in the memory. I am not sure they would be controlled crimp, however they will have ZERO neck downs in the bends, they use internal "oblong" balls inside the tube to keep the internal diameter the full 2.5"

Very good! After watching the video Gary B. posted I can see the substantial neck down that can occur.
Thank you to both you and Gary for educating me on this.