I would say that you have been treating this as a fault which is leading you astray.
With the upgrades the engine is developing more power and thus more heat.
A high effiemcy radiator or core will probably fix the problem.
You may get away with a heater core positioned behind the grill and plumbed into the existing heater take off.

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Cheers Dave.

Our 850 has very similar specs - 1132, stage 2 12G295, twin 1-1/4's, 731 cam, Peak ignition.

It's running a stock rad (don't know that I've ever swapped it in 46 years...), 13lb cap, stock water pump, 82C thermostat, original steel 16-blade fan and 33% glycol mix coolant.

Temp always sits around 82-85. It can see low 90's if the ambient is well up the 30's, but I tend to not drive him in those conditions - open windows just don't cut it for aircon any more I'm afraid!!!

The mini cooling system is a "massive" 3 litres (3-1/2 with heater) so there's not much leeway. I remember when I built the current engine back in the 80's the temp did run a bit high, so I tucked an overflow bottle in between the rad and the firewall. The addition of roughly half-a-litre of coolant to the tiny cooling system made all the difference.

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Cheers,
Rod.
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'63 Morris 850 (Old Secondhand CAR)

Thanks again everyone.
I think that I'm going to go down the path of installing a high performance radiator. Looking at the on line catalogues they all state they will fit into a Morris with minor shroud modifications. Is this something that's easy enough to do at home or is it something that I should farm out. I'm relatively comfortable at performing most jobs on the car.

Cheers,
James.

Hi James, should be easy to fit with minor elongating of the holes in the shroud. I usually put masking tape around the body work to protect the paint while you are working out how to make it fit.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

For a "huge" 1-1.2 Litre engine...

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"Show me the Mini!"

Hi everyone,
I think the problem has been solved. I replaced the radiator with a new aluminium one. I took the car for a 30 minute spin down the freeway. The temp held at 85 degrees the whole time.
Cheers,
James.