Is there a bead on the pipe? That is the usual method to improve a seal. Push the hose on past it then clips also past it. The hose tightens over the ridge formed by the bead. It was the norm on rads we made in the factory. Brass tanks and pipes with a bead rolled in on a beading machine. Pipes then spot welded to the tank for strength using a jig to orientate correctly. Pipe expanded into the hole in the tank and high temp solder to seal.
If I remember correctly we assembled Triumph Acclaim rads that had one piece plastic ranks/pipes with a moulded bead.
One tip that might work on some pipes is to use plumbing joint olives to form a bead. If the pipe dia matches olive sizes then try it. It works in domestic plumbing compression joints so a bit of solder to locate it on a car cooling pipe that is ok to solder. A suitable adhesive should work on plastic pipes. (JB Weld or similar).
