Friday, June 29, 2012

PROJECT SR: Refurbishing an Exhaust

The stock exhaust pipe on the SR was quite pitted and showing a lot of corrosion. It turns out not only can you not buy these new anymore (no surprise there) but finding one in good shape is near impossible. On top of that there are no companies that make aftermarket exhausts for this bike.

But why should they really? It's a single pipe into a muffler. Nothing too crazy, but the bends are a bit complex and would be difficult to mimic without a mandrel bender.

I first looked into getting it replicated in stainless via a mandrel bender. It was more then the cost of the bike as a whole. Given that Kara, the owner, had given me a $2000 spending budget, that idea was out the window.

So I started looking into what it would take to bring this exhaust back from the dead...

The SR's exhaust is sleeved, meaning there is a smaller diameter pipe that runs the full length. This sleeve had cracked off from the inner wall and was now freely banging around inside the pipe.

(sidenote: I have no idea how this part is manufactured. Both pipes are somehow bent at the same time but never touch aside from the connection at the front.)

In order to remove the sleeve, I had to make cuts at the quadrants of the bends so it would be able to slide out. Quadrants for those of you who don't spend all day in CAD programs are the beginning and end of a curve.

After chopping the pipe into 3 sections, I began tackling the pitting and corrosion issue. I used an 80grit flap disc to start resurfacing the pipe. I was pretty surprised at how well it worked. Mine was a bit worn so a finer grit may be better.

After cleaning up all 3 sections, I beveled the edges in preparation to weld them back together. This allows better penetration of the weld.

I then tacked the pieces back together, using the visible interior seam of the pipe to line up the rotations. I'm using a mig so the welds are a bit fatter then I'd like to show off. I've also only been welding for 6 months or so ... next step - grinder.

I again used the flap disc to smooth everything out. The dexterity you get with a flap disc over a grind wheel / cut off is like using your hands vs. your feet. They're amazing.

Here's the final product. I hit the muffler with the flap disc also because I hate chrome.
I sent the pipe out to get plated, however I didn't take the finish down far enough to rid all of the pitting. Plating severely eventuated those flaws. I'm going to hit the surface a few more times and have to send it back. I'll post pictures of the finished product when I finally get it right.


  1. they might form the exhaust similar to how they make brass instruments, they use hardened pitch or frozen soapy water to support the tubing during forming then they melt it out afterwards.

    great job though. your site has also inspired me to get a bike. picked up a kz400 and im looking to do a few small changes to get it more cafe styled.

  2. I was thinking something along the same lines, except not frozen - water is an incompressible fluid, maybe something similar to hydro-forming.
    That or a process like yours, MUCCI - working in sections and the final step welding the inner and outer at the exhaust flange (where yours had broken).
    So glad you've documented this build, it's a priceless reference for me. THANKS MUCCI!