I have done loads of research in to this; there are a few main reasons
Aluminium engine - prone to warping
Rear engine - heats up in the back allot
Thermostat location - too far away from engine head, so opens late
Radiator - too small and doesn?t get cooled, like in a front engine car.
I don?t think the centrifugal oil filter helps either, no one ever cleans them.
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All these problems are easy to cure re-torque the head bolts, often.
Making sure your thermostat is ok, as mine was full of gunk, and couldn?t open. Which actually caused my head gasket to go. Some people even take the thermostat out so it always cools. The Hillman imp was infamous for head gaskets & over heating again, an all aluminium rear engine car. My brother and imps, 500 and 126's. Thermostats from hot countries can be used, but a company I have heard of relocates the thermostat to nearer the head so opens much earlier.
The heat can be controlled, not a major issue.
With regard to the radiator Paul with the polish import who's in Ireland, suggested having the radiator made bigger with more channels. Another thing, which he suggested, was have a switch in your dashboard, connected to the electric fan, so you can have it cooling the radiator all the time. Or a larger fan could be used.
The oil filter type, which never gets cleaned, wont effect the cooling, but it doesn?t help lubrication, which in turn will cause wearing and heat.
Could they have had the piston bores with fins on like the air-cooled engines??? to allow heat to escape??
Hope I haven?t rambled on too much, I like that subject as I have BIS.
Regards
Chris