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[smalt36]

Cheers terry wasn't sure what product to go for , its after the sensors so shouldn't be a prob?

[terry paget]

What do you guys do at the butt joint/ flange between cat and 1st section of exhaust, mine just had paste in when I took it apart butt cat came with a crush sort of gasket which I have attempted to use butt I think its blowing, passed mot though!

I use, and recommend, Loctite SI5990 copper silicone high temperature sealant, Halfords £5.99 per 100 ml. tube, gives a flexible high temperature resistant seal.

repeat after me NO SEALANT.
Silicone buggers the lambda sensors. You can use it between the centre sections and the back box if you like but the downpipe joint is too close to the lambda sensor.

So how do we seal between the cat section and the central section? There is no gasket specified or supplied by Vauxhall. I appreciate any sealant will harm the catalyst, but I was not aware sealant would harm lambda sensors.

[Andy H]

What do you guys do at the butt joint/ flange between cat and 1st section of exhaust, mine just had paste in when I took it apart butt cat came with a crush sort of gasket which I have attempted to use butt I think its blowing, passed mot though!

I use, and recommend, Loctite SI5990 copper silicone high temperature sealant, Halfords £5.99 per 100 ml. tube, gives a flexible high temperature resistant seal.

repeat after me NO SEALANT.
Silicone buggers the lambda sensors. You can use it between the centre sections and the back box if you like but the downpipe joint is too close to the lambda sensor.

So how do we seal between the cat section and the central section? There is no gasket specified or supplied by Vauxhall. I appreciate any sealant will harm the catalyst, but I was not aware sealant would harm lambda sensors.

The silicone doesn't attack the catalytic converters directly, It contaminates the lambda sensors which then give wrong readings to the ECU.

With the wrong readings the ECU will consistently over or under fuel which can destroy the cats.

[terry paget]

But the catalyst and the lambda sensor(s) are upstream of the cat section to middle section joint, Andy, so not at risk of contamination.

[Andy H]

But the catalyst and the lambda sensor(s) are upstream of the cat section to middle section joint, Andy, so not at risk of contamination.

How familiar are you with 'valve overlap'?

The flow of exhaust isn't always one way in the downpipe.

[05omegav6]

Manifold to cat has a gasket... No paste required, just the gasket.

Cat.to centre pipe on the V6, some assembly paste if absolutely necessary, but shouldn't need anything more than a decent clean up...

Likewise on the 2.2, although the flexi pipe is usually what fails... This should only be replaced with a decent quality item and should be welded on rather than clamped...

[terry paget]

I am aware of valve timing overlap, I am aware that exhaust pipe pressure fluctuates, mainly because each pot exhausts a quarter of the time. On my beloved Velocette 500cc single motorcycle I could hear every exhaust  stroke, I imagine the exhaust flow pulsed in-out every time. On the OP's 4 cylinder engine I imagine it's a much smoother flow, but still with some pulsing.

Adam Opel must have been aware of this when he designed the exhaust system. He put gaskets between head and manifold, and manifold and downpipe. He did not bother between cat section and middle section because thought it did not matter, leaving owners to do what they liked.

[Lazydocker]

I am aware of valve timing overlap, I am aware that exhaust pipe pressure fluctuates, mainly because each pot exhausts a quarter of the time. On my beloved Velocette 500cc single motorcycle I could hear every exhaust  stroke, I imagine the exhaust flow pulsed in-out every time. On the OP's 4 cylinder engine I imagine it's a much smoother flow, but still with some pulsing.

Adam Opel must have been aware of this when he designed the exhaust system. He put gaskets between head and manifold, and manifold and downpipe. He did not bother between cat section and middle section because thought it did not matter, leaving owners to do what they liked.

He didn't design it for owners to work on, he designed it for a mixture of performance (of the part) and ease of assembly 

[terry paget]

Searching in vain for the recommended curing time for Loctite SI 5990 I discover on the box it claims, in 4 languages, to be oxygen sensor and catalyst safe.

[smalt36]

Yeh I saw that aswell, still not done it yet!

[terry paget]

Its an easy job to do. Squirt a ribbon of SI5990 around the rounded bit of the cat section, insert the screws, torque evenly to 18Nm, then wait. I gave mine 14 hours, it was fine then. I used Holts Gum gum on the central to back box seal, it acts as a lubricant easing insertion of silencer on to central section.

[TheBoy]

But the catalyst and the lambda sensor(s) are upstream of the cat section to middle section joint, Andy, so not at risk of contamination.

How familiar are you with 'valve overlap'?

The flow of exhaust isn't always one way in the downpipe.

On a 2.2 (or a standard 2.6/3.2) the lamdas are so far upstream, it won't affect it

Even after cleaning, sometimes a smear of paste is needed between cat and mid sections.