Expansion Tank Connection


All keeping well i hope amid this mad time we seem to be in!!!!
In lock down of sorts so time for GT40 work......
Expansion vessel I have has 4 connections, one below the cap which is the vent I assume, two just under that one both threaded and one at the bottom also threaded.
So one of the top threaded ones goes to the radiator, is the other to be blanked off? I can only see ones with 2 threaded connections on the web
The bottom threaded one, does that go into the inlet manifold by the water pump which I am sure Mick at SGT said it can or should it go to a lower hose


Lifetime Supporter
I'm not sure what engine you have, but here's my guess:
  • The one just under the cap is the pressure relief which should go to the bottom of an overflow tank so that the fluid will be drawn back into the expansion tank when the system cools
  • The two beneath that, but above the fill line are for the radiator bleed and engine steam vents (an LS has 2 or 4 which are typically into 1 before going into the expansion tank)
  • The one at the bottom should go to the pump's inlet. It's used to fill the system and provide some head if the pump cavitates
Mine is set up as above except that I also have a coolant swirl tank with a vent that connects to the expansion tank above the fill line.


Hi Scott, thanks for the reply mine is a 302 73 block.
I thought the one under the cap was just to vent excess pressure / water I'll check out the overflow tank, maybe I saw them as catch tanks?
Radiator bleed is great thanks I have that plumbed in
Steam vent I don't think I have?
Bottom one that makes sense, used to fill the system at the highest point I guess

Terry Oxandale

Skinny Man
Shaun, I believe it does both. Excess will push up through the spring-loaded seal at the cap's pressure threshold, and then on cool-down, water will be drawn back into the block through that same one-way seal. I added a small manual steam bleeder valve.

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Cheers Morten really helpful, got a bit more time over the next few weeks now, keep safe mate