The stainless steel butt joint pipe clamp and donut gasket I ordered arrived in the mail in time for the weekend, so at the earliest opportunity I climbed under the car to put things to rights.
Unbolting
As I unbolted the broken section of the downpipe from the section with the catalytic converter I noticed a few things. First, the bolts were rusty as heck. That was expected, but it did make it a pain to get them off. Second, someone else had obviously been here before me as the cat is not original Volvo. Thirdly, and maybe most tellingly, the person installing that new cat on did not have the proper sized bolts on hand. All he or she had where ones that were only threaded for about an inch or so from the tip. This meant the nut could not be tightened far enough up to secure the pipes together. To solve this problem this mystery mechanic used bigger bolts as offset spacers on the other side. This effectively moved the threaded section far enough back that the nut could now tighten securely. Expedient but sub optimal in my opinion. Anyway, two of the bolts came off relatively easily but one was stubborn enough that I had to cut it in half. Once they were all off the pipes came apart and the old donut fell out.
Clamping the Pipe
There are a number of options available for putting the downpipe back together. This include welding, using epoxy muffler patch tape, a butt joint clamp, and, most drastically, replacing the entire downpipe. I decided to first try the clamp as it was the most straight forward and easiest. Just wrap the clamp around the pipe and tighten the bolts.
Reassembly
Now that it was time to bolt everything back in place, I had bought a set of M8x40mm fully threaded bolts that the internet suggested were the correct size along with copper nuts. It turns out that 40mm is the correct length when securing fixed flange pipes together. Mine are the type with the spinny flange on the end that necessitate the donut gasket. So in the end I had to replicate this hack with some of my own hardware and two of the original bolts. Two rather than three because one bolt was so rusted together I had to cut it off with my trusty rotary tool. I’ve also ordered M8x60mm bolts which should be long enough and will replace this setup.
Results
With everything back in place there was nothing left to do but start the car and see how loud it was. It’s still pretty loud. I think the clamp was best suited for connecting two pipes that have well finished ends that can sit nicely flush with each other. That definitely does not describe the two parts of my pipe. This means that I’ll probably have to consider one of the other options. I may try my hand at welding it, although I am a terrible welder. Or I might just use the welder to tack the pipe in place and then use the muffler tape to seal up the joint.