Regarding the leaking end cap:
I understand that this is a problem of the D2-55 C, D, E and F.
In our D2-55A's 2003 this problem does not occur.
The reason might be that in the older engines, there is a copper-coloured 3 mm thick plate between the large casting and the end cap, with O-rings on both sides.
Consequently, the salt water
does not touch the large casting, which by the way is an aluminium alloy, not cast iron. And no corrosion
This copper plate is part nr. 6 in this exploded view of the D2-55A and B heat exchanger
Volvo Penta Exploded view / schematic Heat Exchanger, Components D2-55, D2-55B - MarinePartsEurope.com
This is the exploded view of the D2-55 C, D, E and F heat exchanger
Volvo Penta Exploded view / schematic Heat Exchanger, Components D2-55C, D2-55D, D2-55F - MarinePartsEurope.com
One is tempted to think that the change of the heat exchanger design from the C's on was purely done to reduce cost. They saved 2 Allen screws, one copper plate and a few O-rings. Plus maybe some other cost savings.
The problem can not be solved
by retrofitting the old design, since Volvo Penta
made this design obsolete.
Maybe the limited cost to Volvo Penta
for warranty complaints is offset by the out-of-warranty profit of selling spares for this problem. So no rational reason for Volvo Penta to solve this issue.