[steve s]

Yes, the water pump will push SOME water through your heater, but it will do it a lot better if it's working against the slight back pressure from a thermostat. Plus, your water will be a lot hotter sooner with a thermostat; in my experience, without a thermostat your heater will be nearly useless (Michigan winter) even if there is lots of flow. A 180-degree stat will work even better, but a 160 is usable. Be sure your water outlet to the heater is between the water pump and the stat. I HIGHLY RECOMMEND the Vintage Precision thermostat block which has taps for heater hose and temperature gauge. THe block has small bypass holes machined in so it shouldn't be necessary to drill extra holes in the stat. I believe that I drilled a 1/8" hole in mine, just to be sure. The concern is that if coolant flow is TOTALLY blocked during warm up, local hotspots might occur, possibly causing vaporization, overflow blown back out the radiator overflow tube, and motor damage. Also, some flow by the thermostat's sensor pellet is good to be certain the stat is getting a good reading of the average coolant temperature in the head. Such bypass is in addition to that provided by the tiny hole stat flanges typically have to allow trapped air to bleed out should, for example, should any get trapped when coolant is added or changed.