S1: Header tank pipe functions?

[jagwit]

What is the purpose of the header tank pipe connections. The 4.2 I'm helping a friend with has 3 connections:
1) large pipe from engine to header tank;
2) large pipe from header tank to left of radiator
3) smaller pipe from header tank to right (cool side) of radiator

I presume:
1) obvious, feeding hot coolant (once thermostat is open) to the header tank;
2) feeding hot coolant from header tank to input side of radiator?
3) ???? This pipe has me baffled.

[jagwit]

I found this post but sadly the pics no longer show (dropbox again I'm sure). I duplicate here to some extent with thanks to ada5.

Anyway, the text goes a long way to explain:
1) Pipe from engine thermostat housing goes via a pipe internal to the header tank, directly to top-left of radiator. This pipe has a small opening on the side where the otter switch protrudes into it to sense the coolant temp (for fan control) but apparently, does not seal 100%. Small amounts of uncooled coolant and air can thus escape from this opening into the header tank.
2) The smaller pipe from the right-front that goes to the top right of radiator, serves to allow the cooling system to be filled with coolant (replacing air that comes out via the same pipe), expanding coolant into the header tank and provide a path for air trapped on the RH side of the radiator to the header tank. I have read of owners blocking this pipe off....

Aftermarket header tanks caution:
It seems that aftermarket header tanks might have the opening for the otter switch too large which would result in the creation of a permanent bypass circuit for coolant to bypass the radiator: From engine pipe internal to header tank (high pressure), out the otter switch hole, to the RH side of radiator (low pressure) via the smaller pipe and back to engine - rendering the cooling system less efficient.