207 CAT replacement. What do I not undertstand?

[LittleOldSuzi]

I have just changed the CAT on a 2009 1.4. The old one came off easily enough and rattled so it was end of life. Anyway, it is all went back together fine except the flexi connection is blowing. The new cat came with a fancy looking 2 part clamp, a gasket that seats between the 2 pipes and an outer ring that clamps it all down.

Looks great, but the fancy thing leaks. I thought to hell with this and went and got a standard clamp from the local motor factors, it leaks as well.

I have been tinkering with cars for decades and this joint is making me feel silly. So, what am I missing? Is that new clamp needed for some reason? What is this old goat not understanding? I could put exhaust gunk on it but it shouldn't need it. Any advice? Thanks.

[mickthemaverick]

Have you checked either side of the joint for roundness?

[LittleOldSuzi]

Have you checked either side of the joint for roundness?

I am not exactly sure what you mean but the CAT is new and the joint was previously sound, so I assume everything is the correct shape.

I have a 3rd clamp and will try again with exhaust paste sometime next week. Cheers.

[mickthemaverick]

It is possible that the old Cat and pipe join got a knock at some point, taking them slightly out of round but as both parts were affected there may not have been a resultant leak. However fitting brand new on one half might not match the profile of the existing pipe exactly. If you have a pair of lockable calipers/dividers that you can span the diameter of the pipe with then lock that distance and rotate the tool round the pipe to see if that diameter changes, ie the tool goes loose or tight at any point thereby showing it is not a true circle so will not marry up perfectly with the new Cat.

A sketch to show what I mean: (Click on the image to read clearly)

MTMO

[Arkis]

Did you check if the old gasket went away with the clamp? Usually there is soft gasket from some sort of metal that sits between the pipes and you clamp it together.

On the new clamp, the clamp hold the sealing material, but usually it is separate from the clamp. If the old seal is still in place, it will blow because you double layered the seal.