I have a newly rebuilt engine in my 67 round nose, it is a 1330cc with mild cam and Cooper S spec head and twin 1.5 inch SUs. As part of the rebuild it got a new dual-core radiator, water pump, fan, thermostat, etc.

It has an under bonnet heater and the thermostat is 74 degrees. When I start the car and drive it around the suburb it sits at half way for about 15 minutes then all of a sudden goes to almost three quarters of the gauge.

I replaced the thermostat and reverse flushed both the radiator and heater. I also bought a radiator cap with a thermometer in it, it shows a peak of about 94 degrees.

So, why is it overheating? If the heater was blocked, would it cause this problem? Could the radiator be blocked already, or maybe dodgy from new? Is it possible there is an air pocked in the heater when I fill with coolant?

Are you sure your fancy radiator cap is the right length for the radiator?

At 94°, it shouldn't be boiling as coolant + 13psi (or 16psi) of the radiator cap pressurises the system raising the boiling point past 100°.

Have you checked the temperature with another gauge to rule out if the gauge is 'out of tune'?

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"In two years time your car will be like a lady's clothes, out of date, my car will still be in fashion when I am dead" - Sir Alec speaking to Pininfarina

Yeah firstly I would be suspect of the gauge. Just because it reads 94 doesn't mean it is 94. If the radiator isn't leaking out coolant and boiling over it's technically not overheating.

I personally run a 84 degree thermostat and run my car at around 93 degrees normal temperature. Mine never overheats to the point of boiling, it just hovers around the 93-95 degree mark in heavy traffic (I mean 30 degree day for 1h 45 mins bumper to bumper) and I don't give it a second thought... Every other day, especially cool nights at speed, it never goes past low 90's (where I want it to be) and some nights the thermostat doesn't even open

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Lillee - 1969 Morris Mini K

Maybe your temp sender in the head is dodgy or the wrong one for your gauge.
If the new cap gauge is reading 94 and the cabin gauge is at 3/4 then it sounds like your inside gauge is reading high.

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82 BJ42 just completed resto
64 850 Modified(about to start rebuild)

I had a similar thing on my Cooper S.. It was reading between 1/2 - 3/4, got it home, and it was sitting on 71deg C... Changed the Temp sender and all appears fine

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Is your fuel gauge playing up as well ?, could be the voltage stabilizer .
Do you run an electric temp gauge ?

As others have said , try another gauge , even borrow a working capillary gauge and see what it tells you .

You should be able to give your heater core a flush , without running it through the motor to make sure it is clean .

It is not using any coolant, the temp gauge is new, the sender unit is unknown and the voltage stabiliser is unknown.

I would have thought at approx. 94 degrees three quarters up the gauge is about right. Is this not so?

After about 15 mins of driving the gauge was at three quarters, when it cooled down the coolant level was just below the top of the fins of the radiator. Is this normal?

No.
It is normal to lose about 1/2 the top tank due to expansion when the water heats. That's why there is a `fill level' indicator inside most Oz Mini rad top tanks.
If losing more I'd say your cap is incorrect, or else there is a mark or crap on the seal lip inside the neck. Clean it up with some wet/dry paper until shiny. There is a ^ ring right around, which seals to the cap.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The best thing to do is get an electronic thermometer, about $20 from Dick Smith etc with a metal probe.

When engine is cold, open the radiator cap and put in the thermometer, start her up and watch the thermometer reading versus your gauge.

"3/4 gauge" reading can mean anything if your gauge is not right, your sender is not right or your regulator is not right, or worst still all three!. The gauge is about as trustworthy as Lance Armstrong (maybe slightly more trustworthy perhaps).

Only then can you tell if it is truly overheating or not.

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Lillee - 1969 Morris Mini K

Lillee, you obviously having been near a dick smith for a few years!

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1975 Clubman
1152cc, 40dcoe weber, RE83 cam, GRed head.

Lol ok ebay

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Lillee - 1969 Morris Mini K

Jaycar... They have a good laser digital thermo for ~$50.
http://www.jaycar.com.au/productView.asp?ID=QM7215

Or for less you can get a DMM with temp probe as Lillee suggested-
http://www.jaycar.com.au/productView.asp?ID=QM1323

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

i got one of these http://www.jaycar.com.au/productView.as ... ATID=947#1
very handy