updated 16 september 2019
this section covers the 195.6 OHV cooling system, what i believe is a definitive explanation of this engine's chronic headgasket failure/overheat problem, and and easy fix.
this engine also silently suffers from lubricating oil overheat issues -- the "lower end", crank, rods, cam, tappets, oil pump and oil galleries, are unchanged from it's 1940's introduction as a 75 horsepower engine. a stock engine cruising at modern highway speeds, the engine oil temperature becomes a problem. this is a harder problem to solve. oil cooling is covered in the lubrication section.
any radiator relies on the temperature difference between inside (coolant) and outside (air). a high heat load (climbing a hill) in winter is not a problem because the difference is high (cold outside); conversely hot weather decreases the inside/outside difference, as is intuitively obvious.
low thermostat temperatures (160F) lower this difference, and the thermostat is usually open, trying to remove heat even in only moderately warm weather. in hot weather this only gets worse, and engine temperature rises. even when the rise stays within safe operating range, fluctuating temperature changes operating conditions (carburetion, combustion/ping issues, oil film thickness) and accentuates the thermal issues described below.
i strongly recommend the factory recommended 190F operating temperature (thermostat). this makes for a generally large inside/outside temperature difference, hence better regulation.
run any engine long enough, something fails first. on this engine, it is the head gasket. Nash/AMC knew there was a problem right from the engine's introduction: the technical service manual specifies a 4000 mile head bolt check/retorque schedule, and with the engine hot. the alleged reason is bolt torque. i am now convinced it due to head bolt motion.
poor thermal coupling between cylinder head heat and the thermostat is the root cause of a complex stress mechanism. the thermostat is isolated in a pod in the head well forward of #1 cylinder. with the engine "cold" (first start up of the day) block and head are the same temperature. when the engine runs, combustion heat accumulates in the cylinder head. keep in mind that there is no coolant flowing (thermostat closed), and that the headgasket is a heat insulator. the thermostat, some four inches forward, remains isolated from combustion heat.
the thermostat isolation delays the heat signal from reaching the thermostat. the thermostat can only get the heat signal via simple conduction/convection, or via leakage in or around the thermostat itself. my measurements show that the delay is so long that the coolant in the hot parts of the head exceed boiling, with audible steam-hammering, before any of that heat reaches the thermostat.
when the heat signal finally reaches the thermostat, and it begins to open slightly, coolant flow moves the now extremely-hot coolant from the upper head past the thermostat, which immediately opens fully. cold coolant then flows up into the head from the block (and radiator outlet). the hot metal that had been boiling head water when the thermostat was closed is now bathed in relatively cold water.
with the thermostat open the temperature stabilizes normally. however this is preceded by cylinder head severe overheat, followed by overcooling, and it is this temperature cylcing that causes the head to grown in length (hot) then shrink (cold).
i measured coolant temperatures of over 250F, accompanied by audible steam hammering; at the same time the block remains cool to the touch. i estimate during this time that there is a 150F degree temperature difference between block and head. assuming 150F difference, i calculate 0.024" cylinder head length increase (heating) and decrease (sudden cooling) in these first few minutes. the head gasket is a thermal insulator and "lubricant" between block and head.
given this thermal cycling and expansion/contract it is not hard to visualize the undesirable horizontal motion of the head bolts. when the head grows in length the head bolts splay out in a "V" with the bolt heads moving apart; when the head and block temperatures equalize, they move back to their correct vertical position. i believe this back and forth motion applies rotational torque and backs out the head bolts. the expansion/contraction is likely bad for the sealing surfaces, contributing to leakage. accumulated over time this loosens the head and causes the leaks that are symptomatic of the common end-of-life failures in this engine. if you think this bolt-loosening theory sounds dubious, check out this page at BoltScience.com: the Jost Effect. there's even a video showing transverse motion backing out a bolt!
as serious as the cold-startup problem is, the fix is laughably simple: drill a 1/8" bypass hole in the body of the thermostat, install the thermostat with the hole towards the front of the car, so that it "leaks" coolant past the sensor button.
placement isn't critical, but the hole wants to be inside the gasket and housing area, and don't damage or nick the center portion (that opens; click the photo to see the slightly raised center with the spring inside it).
this also helps purge air from the system. newer aftermarket thermostats often have a hole and a loose pin so that crud can't block it.
i suspect that many thermostat installations leak slightly, by design or by accident. this might explain the disparity in experiences (some have head failures, but many don't). i suspect engines with constant if small coolant flow do not have this thermal-spike issue.
since 2016 i have run an all-electronic cooling system that uses two small
electric pumps (no belt-driven pump, no thermostat) to control
engine temperature. this electronic close-loop
cooling system is a project unto itself and is described elsewhere. the
basics described in this page are still relevant.