There are many problems with the plastic set up.
1. The thermostat housing often leaks from the housing, around the plastic weld. That weld at times is not water tight.
2. The plastic pipes and the curved pipe have a seam in the middle, along the length. They tend to split here, and can open under pressure, only to close again when cold. Exactly like the T-piece on the 1.8T.
3. The groove for the O-ring is squeezed against the aluminium bore, on the one side, and the base of the groove in the plastic, on the other. When new, the o-ring will have a certain amount of elasticity, which will be able to follow the heating and cooling of the engine, and thus seal, by maintaining a pressure all around.
When older, two things happen. The o-ring get square and looses elasticity, and the plastic base of the pipe bulges under pressure from the o-ring, and gets permanently deformed. Eventually the o-ring can no longer exert sufficient pressure, and the system starts to leak.
There is no good reason to use the two locks on the straight pipe. Once the pipe is in place, it will not move, as there is no pressure differential on the two ends of the pipe, iow, nothing is pushing the pipe one way or the other. The lack of the two locks would in any case have no bearing on the curved pipe, which is under discussion here. So that is just a distraction in this case!
O-rings can be had in many sizes. To compensate for rust or wear in the bore, make sure the bore in the aluminium is cleaned to bare metal, and look for an o-ring slightly thicker than what you have used, or, if you can't find that, then slightly larger in diameter, which will give extra material to compress the o-ring once installed.
The problem with plastics as mentioned above will however prevent any solution involving plastic from being permanent.
That is just a simple scientific fact!
