I have a mid 80`s +8 with a plastic expansion tank which has a very definite water level marked on it.
Over the years I have been on Morgan forums there has ever been chat cropping up from time to time on expansion tanks and their purpose, water levels, types of cap and what cap goes where.
I suspect if you visit GoMoG you will find plenty of detail.
I suspect the height that the expansion tank is fitted relative to the rad top tank may be important. Also that the spring loaded pressure cap is fitted to the expansion tank and not the rad, the rad cap having no spring and just containing a rubber seal in order that expanding and contracting water can move freely between the rad and expansion tank as and when required.
As for coolant levels, on my +8 when cold the level in the expansion tank sits right down at the level of the tube that connects to the rad, and after a run the coolant has expanded into the tank and sits at or near the level indicator arrows moulded into the tank.
Whether any of the above might be considered accurate or not...? All I know is that my car with standard rad and standard engine spec has never overheated even when Italians were fainting in the street due to heat exhaustion when we toured there in the middle of Summer some years back.
Good luck in your choices..
