Hi Nibsy
Don’t feel bad about the money, I’ve been getting shafted all my life with car and bike parts.
In the UK there are plenty of manufacturers and distributors that throw out complete crap, knowing that there are idiots out there like me that amend things to make them work.
I’ll need to start taking photos and notes of the work I do on my vehicles as my memory is poor, I done this job just last year and it’s only starting to come back a bit now.
I’ve read the couple of pages on your issue and the information is all there. When I worked on this exhaust I noticed that the strength for retaining the rear pipes comes from the clamping onto the collector box lower down, and the top section with the clamps and copper ring were fairly slack and could be pulled apart with a jerk. As the top pipe slides partly into the mating side, then the full force of the exhaust gas will not hit the copper ring, but in your case it appears (as above) the copper ring is not thick enough to hold back the low level gases pressure. Can I suggest something that has been already said, fit a shim ring to make up the distance that the copper ring is not taking up, If you cannot find one, there is a good chance I could make you one out of mild steel if I have some dimensions and sizes to work to.
Again; good luck.