Is household silicone any use as an exhaust joint paste?

No. Mike.

I've heard of it used but it needs to be downstream of the cat, if fitted, as apparently the chemicals it releases can damage it.

Yes, clear is best.

We seem to have a difference of opinion. I'm sure I've seen my mechanic using something that looks suspiciously like silicone with a skeleton gun on a back box joint (no catalyst).

Well the way I see it is, if it works on the exhaust manifold to head, it should work anywhere on the exhaust system.

The message from "ThePunisher" contains these words:

I've seen it on inlet manifolds...

Where is it's use recommended for exhaust manifolds?

Even those designed for high temperatures, can only take around 300 C. That might be above the temperature of an exhaust manifold at the joint with the head, but I doubt it will be high enough for joints further down the line, and that is for a high temperature silicone sealant. As well as that, the OP is asking about using a household silicone. which will not withstand anything near 300 C I certainly wouldn't recommend using any silicone sealant on exhaust systems, so AFAIC the answer is still no. Mike.

I would think it's more likely to have been an exhaust assembly paste/sealant. Mike.

The message from "Mike G" contains these words:

The last time I had an exhaust done it was exactly that. Just like the stuff you buy in a toothpaste-type tube, only in an industrial cartridge.

The stainless steel fabricator I got to fix my system used silicone throughout on the SD1 - including manifold to downpipes. I suspect it is a special for exhaust use, though.

AFAIK even the best automotive high temperature silicone sealants are only suitable for temperatures up to around 340C. I think downpipes easily exceed those sort of temperatures. My turbo Celica certainly does. :-) Regardless, I think I'd rather stick with more conventional sealants. Mike.

Rated up to 950 degrees F = 510C. Specifically mentions exhaust manifolds in the blurb.

Looks like it's high temperature silicon with some ceramic fibre and powder mixed in.

I'm well aware there are sealants that can withstand very high temperatures. Your link simply points to one of them.

There is nothing whatsoever in the press release to suggest Pyro-Putty 950 is silicone based. It's clearly resin based. "combining a unique high temperature resin with various ceramic fibers and powders" Seems pretty clear to me. Mike.

My apologies - Google pointed me to this one but took me to that one!

"Ceramic-Silicone Sealant suits applications to 1,100°F. "

This one doesn't mention exhausts though.

This related one does mention 'automotive use', and is definitely a Ceramic-Silicon mix.

I'd tend to go for classic exhaust paste anyway, as it happens.

I wonder if the use of silicon sealant at exhaust fitters is in any way related to it failing a few years down the line earlier than paste would?

Any silicone content is probably very small in both those products I think they're just trading on the term 'silicone sealant' because it's one that is familiar to most potential buyers. You may do better, but I can't find any silicone based sealant that can work above 360C, and that's only intermittently. 340C was the maximum sustained temperature I found. Certainly way beyond the range of a silicone sealant for domestic use.

I've never seen silicone sealant used by exhaust fitters. All I've seen them using is cartridges of std exhaust assy paste. The sort of stuff you can buy in Halfords in a tube. Dries hard, like fireclay. Mike.

Silicon is the domestos of lambda sensors....... kills all known sensors stone dead...... hence why better gasket sealant is usually marked lambda safe.

Quite possibly. I'll ask next time.

That's right. About 260 deg. C.

Wrong. It depends what sort of silicone you buy.

XTS320RTV silicone sealant is good up to around 320 degrees C and beyond.

Shokka