The heater does not work.
The engine temperature gauge never gets more than 1/4 off the cold limit.
The radiator has been replaced, the thermostat has been replaced (twice), the heater core and hoses have been replaced. The heater still does not work and the engine temp still indicates cold (although it will gradually show hotter if you leave it sit and idle for hours and hours.
Why won’t the heater work?
If the engine is not warming up there is something wrong with the thermostat.
While it’s unusual, it’s not impossible to get two bad thermostats in a row.
I’d try a thermostat from a different supplier.
You also have to make sure there are not air pockets in the cooling system.
You’re either doing something wrong or the engine is so loose it won’t generate any heat. I believe on that one there should be a rubber gasket around the thermostat plus a rubber O ring on the housing. You sure you’ve got both? Sometimes you need to ask for them and don’t always come with the thermostat. Just guessing here but if you missed the gasket around the thermostat it may not leak but think you could get coolant bypassing it. If its just too loose, you may have to put cardboard in front of the radiator to stop the heat loss.