Although I don't have a Magnex, mine has the exact same issue, it leaks from the center pipe > downpipe union. Mine is mainly down to the fact that the clamp I used has been used about 60 thousand million times (couldn't be bothered getting another) so although I was exhaust gummed out my nut, it still leaks. I will whack a new clamp on it next time I have it up on the ramps and hopefully it will sort out my issue.
I don't know how your fitting it exactly, but the best way to do it is fit everything on, apart from bolting the downpipe > center pipe, but tighten up everything else. Then rest the exhaust on a prop stant and lower the car until the exhaust sits flush under the body, then push the center pipe as hard onto the downpipe as possible, and clamp it up. That way, it shouldn't sway, twist or pull away (as its the last component to be fitted).
Also, when working with exhaust gum, its best smearing a lot of it on the inside of the clamp, then run a seal around the outside when its fitted. No need for tonnes, just a nice airtight seal. Make sure you are doing it when the engine is running too - it helps identify air leaks, and the exhaust sealant relies on heat to set - but be careful, when the engine is running, and your smearing it on, it will be seriously hot and you should know how sticky it is, it will burn the crap out of your fingers if you don't wipe it off fast.
Ben
