[DanEC]

Then you may have to expand both sides to get them round and a tight slip joint to fix it. You can try the red high temp RTV. it's used very often to seal headers to the head. Not sure how well it will work on a slip joint however. It will be more flexible after curing. As you have seen, it's a problem using sealant on a slip joint as the action of sliding one pipe inside the other scrapes it off. I would try applying sealer to both sides which will make a bit of a mess on the outside of the sidepipe receiver, but it can be cleaned up with Prep Sol or something similar. Applying to both sides should give better chances of getting enough in the joint to seal up.