Overheating

[Kevin H]

Claude,

A freshly rebuilt engine will often run hotter initially due to tighter tolerances, but I don't know how much hotter.

When you say the engine is boiling, is it expelling hot water from the expansion tank overflow - what are the symptoms?

In my experience, when an engine overheats that quickly it has always been a faulty thermostat, (that's not to say it couldn't be something else though!)

Are you sure the thermostat is opening as much as it should? You could try running it without the thermostat and see what happens, or of course replace it. They're not expensive.

If you run it without a thermostat it will take longer to warm up, but it will still overheat if there is another problem.

Another thing to check, is the pressure rating on the radiator and expansion
tank caps. The water system does need to be pressurised. The cap on my radiator is 15psi /100kpa and on the expansion tank it's 13psi/90kpa.