We have not sealed them and due to the thinness of the pipes I don't think a clamp would help. It is a thermal fit and it is very tight. Two seasons on one sled, one season on the other. They have both been through the shop for this season and neither sled has any leaking going on.
I think variations in the pipes and collars as well as the installation could introduce possible leaks and in that case a high temp sealer may help.
They do NOT need "pounding on" but more of a pressing. When the temps are right there is enough expansion and contraction to allow a press fit.
We freeze the pipes and torch the flanges and they press right on. after they cool, they are never coming off. We used a block that we made to position the flanges upright so the pipes could be pressed (by hand) down onto them, this way the depth is controlled as well. The flanges can end up +/- 1/4" from stock without issue as long as they are equal to each other.
Once they are married together, from that point on they heat and cool equally, so the thermal bonding stays. For a long time this type of thermal fit has been used in industrial applications like this when you have two dis-similar metals that can't be welded or bonded in a traditional way so I would expect them to be very reliable (certainly more reliable than the stock flange).
