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4.6 GEMS with SU manifold cooling circuit

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Please be assured my low post count is due to epic levels of forum lurking, rather than me being a weird internet ghost of otherwise weirdo. lol...


My 4.6 GEMS MS build has been value engineered down to carbs & MJ for the time being - to be fair this does suit the 80" it's going in ;)

Anyhoo, I was all for retaining the thermostat in the 2" SU manifold I've got hold of, and making the 4.6 cooling circuit work per traditional style..  However... upon changing the water pump just now I realised there is no bypass return into it, nor is there a heater return? (was going to tee into bottom hose but would rather not make a DIY joint into a hose if I'm honest).

So... I was thinking how the 'cows udder' external stat comes in different temp ratings and things, and does solves the above return issues so I was thinking to use it. I can only imagine it has a bad rep because it's plastic and eventually fails? 

Therefore, above stuff considered, I have sketched a circuit diagram and was wondering if anybody has experience if I am on the right track?


I have looked into this a lot and not found a decisive scheme hence my post which probably seems like replaying the same record from others, for that I am sorry..

Few points;

1. Intend to blank the rear coolant fitting on the carb manifold - is this ok? There is a steel fitting on the front of the manifold, which I think is best for heater flow?

2. The carb manifold i have does not have a bypass port on the original thermostat housing - I will source one, assuming there is a corresponding hole in the manifold (carbs/mani was built for an MGB build by the PO)

3. As diagram, a 3-port cows udder thermostat from disco/P38 (need to check which), will solve both the heater return and bypass returns to the pump inlet.

4. With this arrangement I can valve off the heater circuit rather than use it as a bypass circuit, which is handy as I have the Smiths flat panel 'shin burner' style heater core which goes in the cabin and i will be summer driving mostly!

4. I might even be able to re-jig some original pipes with this layout!

5. I will get some early type rocker covers which have correct breather connections for the SU manifold :)



4.6 GEMS with SU manifold cooling circuit.jpg

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I'm not sure the external thermostat like on the P38 and D2 comes in different temp ratings. For the P38 there is only one partnumber for GEMS or Bosch. The one in the D2 is rated for the same temperatures (both for V8 and TD5), but only has 3 connections (1 bottom and 2 top) instead of 4 (1 bottom and 3 top) as on the Range. That is because the heather return is piped directly into the feed hose and not via the thermostat.

If you plan to shut the heater off completely, I don't think it really matters if it is connected to the front or rear. If there is a permanent flow through the heater, like on modern systems, it might be better to leave it on the rear to ensure a good flow throughout the heads and manifold. And yes, I know the later V8s in fact don't have that rear connection so it shouldn't matter... 

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