Wednesday, August 28, 2013

Sand Bending a CB750 Exhaust

Today I started reviving an old technique lots of folks like to talk about but hardly anyone can actually do!  This technique, known as sand bending, has been around as long as the need to bend pipe (basically since steam power started!).  It used to be pretty common practice among hot rodders back in "the day" but has been mostly forgotten since mandrel bending machines and prefab aftermarket exhausts became very common.  There was a time when a garage builder had no other option to make the smokestacks for his souped up roadster, and since sand bending isn't especially expensive, it certainly fit the bill.

In my case, sand bending wasn't preferable due to cost, frankly, buying pre-bent stainless sections and welding them together isn't all that pricey and is a hell of a lot easier!  I chose to use sand bending for the project of the day because it lets me build a seamless pipe (better for performance) and the quality is simply far better than what's available from mandrel bending shops now - the last time I ordered mandrel bends, they came scarred from the machine's clamps and I was told that's basically par for the course and if I got better, I was lucky.  That just wont do!

Since I have used this before on other cafe racers (the cb350 spitfire, a few cb400f exhausts), I knew what I liked, and what I had trouble with.  What I liked was the way I could create any radius bend and make the pipes fit like a factory setup.  What I didn't was that it's damned hard to freeform 4 pipes and get everything to match up.  Something had to be done before my next attempt.

Since I am building a 4-1 pipe for a cb750, I know I'll have to make the turns from the exhaust port all line up especially well because that's a really visible area that would just look bad if it wasn't tip top.  I also know that the pipes won't be a simple plane, there will be bends coming off in different angles if it's to work right, so I decided to devise a good form and clamp jig to keep my bends consistent as well as to let me twist the tube where necessary.

I tossed up a chunk of aluminum on the good ol' bridgeport machine and turned it into a softjaw clamp that matches my 1.5 OD stainless tube just perfect, clamp down the 4 10mm screws and the tube can really be bent to hell and back without slipping in the clamp and especially without scarring the tube by using a simple vise clamp.

The radius form comes from a piece of 5" pipe I sawed up and welded together, when all's said and done, this jig lets me form a whole 90 degree bend the same way every time, then reclamp the part to form the second turn where the pipe goes into the collector, any intermediate twists and bends are also possible by either holding the clamp in the vise and freeforming, or reclamping on the exhaust pipe in a different area and using the jig again.  It was at least 3 hours work to build, but it's a tool worth its weight in gold for making custom pipes.


  1. Thats amazing... I am going to have to make a tool like this, thank you!

  2. I was just down talking to Sean.... that technique is very cool

  3. That's a real nice job. I'm sure it wasn't as easy as it looks so well done.