I mentioned that I had gotten the parts to add an external thermostat to my 3HM. Well last weekend I installed it after realizing I could simplify the installation and could always go back fairly easily. The thermostat is the very nice bronze casting in the photo and was originally designed for the Atomic 4. It uses a easily available NAPA thermostat. It is attached to the coolant outlet of the exhaust manifold. I reversed the exhaust manifold coolant flow from front to back to back to front. There is a bypass from the thermostat to a T at the inlet of the coolant (rubber impeller) pump for fast warm ups and even temps. The system seems to be working great. I was confused for a while as my temp gauge was showing 160° with a 180° thermostat. I figured out that my temp sender is installed in the head and the thermostat is seeing coolant coming out of the exhaust manifold. Reading temp at the sender I see 160° but at the fitting going into the thermostat it says 183°.
I will give a shout out to my digital temp gauge as we're on the subject. Made by Cruzpro it is very accurate and includes an alarm.
CruzPro T60 Digital Water Temperature Gauge and Alarm