One thing some of you might be overlooking regarding the Petter type engine is WHERE the engine cooling water enters the engine. This has a direct relationship to its warming up. For example, when the petter is cooled by run through water, the water is not put into the engine at the base of the cylinder. The Indian petters have 2 water ports on the head, a high one and a low one. For the run through cooling system the bottom port on the cylinder is capped and the inlet goes to the lower head port. The water surrounding the cylinder is out of circulation except by convection currents. This would allow it to warm up and still not overheat since the head is being cooled and the cylinder water could "thermosiphon" up to the head area. This might be something to try and see whether it helps with a thermosiphon cooled engine. It would take mounting thermometers in each of the ports to really see what is happening. Or possibly one of the new infrared thermometers.
Mike