I have a 1957 model D6 9U that is using or loosing water. The engine oil shows no sign of water contamination so that may not be the head gasket ?? I am thinking it could be pre-combustion chamber seals. How hard is it to replace them if that is what the problem is.
Top off the radiator and check for bubbles while running engine. Also signs of water being pushed out the overflow when under pressure. Do you have visible water leaking out around the pre-com chambers? Any water vapor or smell of burnt anti-freeze in the exhaust? Changing seals means removing the pre-com retainers if original type or removing the pre-com chambers if they are the 1-piece type.
Check your water pump weep hole.
Thanks for the advice there Old Magnet. the are no visible signs of bubbles in the radiator water with the motor at operating temp nor is there any sign of water in the engine oil, its as black as midnight !! I think its a leaking pre-com seal or at the worst a cracked head hope I,m wrong about the head Will keep you posted
Other leak candidates are pin holes in cylinder liners and/or in the pre-combustion chambers.
My money is on leaking precom seals !!