There is no oil cooler fan belt on the 139. The fan is driven off a drive quill and shear shaft directly from the MGB. None the less the shaft shearing, or the fan coming apart could explain the high temp and "grinding noise". On two occasions in the past 5 years I have had hums indications of oil cooler drive shaft problems requiring replacing the fan drive shaft. All in all it is to early to speculate the problem and I for one will be looking forward to learning what happened and ensuring it does not happen again.