Rover 10 gets overheated

[peeren]

We have a Rover 10 from 1937 and have overhauled the engine. We now however have a problem with overheating. After 10 minutes of fast idling the water already cooks.
The cylinder bore looks good and also the cylinder head shows no signs of leakage. Of course the head was milled and a new gasket fitted.

Does anybody have any recommendation of what could be wrong here?

Thanks,
Frits & Wil
The Netherlands

[Phil - Nottingham]

If you have a thermostat fitted it may have stuck? Remove & test in hot water

Water pump impellor/shaft sheared? look for signs of circulating water in the rad when engine running fast

Radiator core blocked/scaled up

[47p2]

Agree with Phil, also it could be airlocked