The heat box around the exhaust pipes rattles and is loose at the front end on the pipes. Any reason not to take it to the muffler shop and have it welded back down tight?

I seem to recall that they weren't welded when fabricated as the constant heating and cooling would cause a weak spot, and Porsche didn't want to have their customers asphysiated by carbon monoxide.

I'd live with the rattle, or have just the outer envelope welded, but do not weld the outer envelope to the exhaust tube itself. Otherwise, if you can find some SSI heat exchangers, that would be the best long-term fix.

I have considered the same thing...Or maybe just some new headers!!

Mild steel HE's appear bonded by friction to the exhaust manifold pipe. A rattle there would seem to indicate failure. When mine rusted through, I peeled them off the pipe and now have "headers" but no heat. I found some SS HE this Summer. These do seem to be fully welded.

The hot air produced by the HE is routed to the cabin through distribution boxes via a pipe and plate assembly that is held together with screws. Perhaps that's what's rattling?

Looking under the car, I can see that welding the hot air routing assembly would be a bad idea for a number of reasons. I also would hesitate to weld the seam between the HE and the exhaust pipe -- mainly because it would be difficult to produce a reliable weld there. I assume (hope) the welds of the SS versions started out as clean metal and were designed to accommodate repeated expansion, heat and shock.

They aren't welded.
CO kills and even the tiniest crack from a weld could eventually split wide open.

Bad idea ...

On the factory heat exchangers I had before, what I did was cut some sheet metal that was the approximate gauge equal to the gap between the box and the pipe and maybe cut them about a 1/4" wide and then basically make two 1/2 circles and tap them just under the edge where the gap is and I would then just tack weld the shim to the sheet metal only and not the pipes themselves. That way it would fill some of the cracks and the box and the pipes could expand and contract separately.

Were you patching fissures in the HE shell or gasketing the junction between it and the exhaust pipe, or both?

If it is the HE shell that's rattling against the pipe, you could try alumina oxide (refractory) tape, either as a gasket between the two or a wrap around the whole assembly. Not much room up there, however.

I suspect that the exhaust pipe expands quite a bit more than the HE shell in operation, closing a small gap.

I basically filled the gap around the pipe. If you get the right thickness for it and just tap them in so they are tight, you probably wouldn't even have to tack weld them in place. Just roll them to the same curvature as the pipe.

That sounds good. yep it's the rattle of the heat ex on the pipes. I'm going to try the spacer idea first...and no welding. thanks

Plus it will keep more of the heat in where it belongs, which is the whole idea.

I had good luck with an old rusty pair by going old school: Brazing up the gaps. (Need a gas-welding smilie!)

I only used them for about 4 years, so I can't say for sure that the low heat did not eventually cause the pipes to crack after I sold that car, but it was fine the whole time I had it. I did have a CO detector in each of my cars during that time... (Get a fresh one every year from a pilot supply.)

Oh, and the rattle my car made was actually the cap on the heat shutoff valve over the bypass outlet. It's a bell shaped piece that's held on by three tab-and-slot things. If the tabs work loose, the bell starts to ring...