Series 1 4.2 header tank

[Curtis Bay]

I am in the process of upgrading the cooling system on my 4.2 series 1 FHC (1965) and I am confused.......

I have studied the design of the header tank and I can't work out what is going on. I understand the flow path of the coolant but I can't see why the main hose that returns coolant from the engine to the top of the radiator appears to run through the tank without a break. Looking inside the tank there is a metal pipe that joins up the two main hoses. On a 3.8 I believe the internal pipe was not present and this allowed the expanding coolant to accumulate in the tank. On a 4.2 engine the expanding coolant can get into the tank via the smaller diameter hose from that runs from the front of the right side of the tank to the the top right hand side of the radiator.

I have read that the pipe that runs across inside the 4.2 expansion tank has some breather holes in it to let air trapped in the cooling system escape in to the tank and that it also has a hole in it to allow the otter switch to measure the coolant temperature as it flows back to the radiator but without cutting up a tank I can't confirm this.

Can anyone help because I am keen to undersatnd what is happening inside the tank?

[ada5]

Curtis,

I know it has been sometime since your posting but I have only just been in a position to add to your query.
I have a series 1 4.2 and it had a steel header tank which had become porous in a number of places.
I decided to obtain a brass replacement to prevent further corrosion problems.
Here is my understanding of how the system works.



The large diameter pipe does go straight through the tank directly connecting the thermostat housing to the upper left side of the radiator. There is a hole in the pipe to allow the otter switch to poke through with a small amount of clearance. See the image below



View of otter switch poking through the pipe.



The smaller diameter outlet connects the tank to the upper right side of the radiator.
When the thermostat is open, hot coolant from the engine is passed through the tank from the thermostat housing to the left side of the radiator. It then passes through the radiator (cross-flow) being cooled on the way. The 'cooled coolant' is then drawn up from the lower right hand radiator outlet and pumped through the engine. The upper right hand radiator outlet connects to the header tank allowing for expansion and filling of the system. The small diameter pipe below this allows water to bypass the thermostat and the radiator when the engine is cold until the thermostat opens.
Be sure you have the correct type of thermostat fitted as a collar moves across the bypass channel when the thermostat opens, shutting off the bypass. The common form of thermostat fitted to most other cars in the sixties does not have this collar and if fitted will allowing some coolant to bypass the radiator even when hot reducing the cooling efficiency on the system.

Going back to my old steel header tank which is a repro. not the original. The pictures below show the large dia. cross-through pipe is set a bit further away from the otter switch meaning it doesn't poke through pipe very much leaving a greater clearance around it. The small amount of clearance I think should allow the air to escape and purge the system when filling with coolant. The larger clearance of my old tank is too much and would allow another path for coolant to bypass the radiator further reducing the efficiency of the system.





Hope this is of use.

David