updated 22 mar 2020
The cooling system for this engine is adequate with two exceptions: a cold-warmup problem and inadequate lubricating oil cooling. The cold-warmup problem can have serious side effects, but is extremely easy to fix. The lubricating oil cooling issue is more complex and discussed in the lubrication section.
The most common cooling system problems on this engine nearly always stem from advanced age and lack of maintenance during it's lifetime. The information on this site assumes you have an engine in basic good operating condition. The Rambler Technical Service Manual is the place to begin diagnosis of a non-working system.
The problems and fixes below are reliability and indirectly performance increasing, to overcome designed-in cooling system problems.
In 1964 AMC incorporated changes (to pump and head) that solves these problems. But given how much engine and parts-swapping was done in these engine's lifetimes, it wouldn't hurt to understand what you have installed.
A design flaw in the cylinder head, outlined in detail below, is responsible for many of the reliability complaints about this engine. This problem has two fixes, one of which is do-it-at-home simple. This fix should be applied to even driven-once-a-month cars, unless they are 1964 and up.
Even if you have a 1964-up engine, you might want to look at and understand the second fix for pre-1964 engines. That fix would allow you to run the earlier, more common water pump on the later engine, shoud the later pump become difficult to obtain.
The cooling system is quite ordinary. The belt-driven pump draws coolant from the bottom of the radiator, pushes it into the front of the block, where it flows past and around all six cylinders picking up heat, then flows upwards through passages into the cylinder head, then out the top-front. The thermostat is placed in the outlet to the radiator where the coolant is hottest. Air in the system (eg. missing coolant!) collects in the top of the radiator. Pulling from the bottom of the radiator makes the pump self-priming (as long as the coolant level is above the pump's vanes).
Thermally, this is a closed loop system. The firing cylinders produce a lot of waste heat mostly in the head. There is a loose synergy between engine RPM and cooling system operation, where "more" heat is produced at higher RPMs, when, through no coincidence, the coolant pump is spun the fastest and moves the most coolant (greater cooling capacity). Assuming that the car is moving, there is simultaneously maximum air flowing through the radiator, helped and sometimes hindered by the fan on the front of the pump.
Since coolant flows from bottom to top, and the combustion chamber water jackets in the head produce most of the heat, the thermostat is located after the hottest part of the engine. The thermostat is a proportional valve, a restriction to coolant flow. It varies from closed to mostly open, depending on the temperature of a little blob of wax inside it. The rated temperature is the temperature at which it just begins to open.
To remove heat, the 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 air outside); conversely hot weather decreases the inside/outside difference, as is intuitively obvious.
The AMC-recommended operating temperature/thermostat rating for this engine is 195 degrees Farenheit. Lower temperatures worsen cooling system problems by lowering the temperature difference, lowering the radiator's ability to shed heat.
Low thermostat temperatures 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.
The pre-1964 195.6 OHV engine has a designed-in flaw that is now known to be the source of many reliability and cylinder-head problems. AMC made a design change in 1964 that resolves this, but earlier engines require your intervention.
The flaw is that the location of the thermostat prevents it from easily "seeing" heat produced in the engine when cold. Under certain circumstances combustion chamber water jacket temperatures skyrocket, creating steam pockets, until heat "signal" can reach the thermostat, located six inches away, and cause it to open.
A side effect of this delayed signal is loosening of the cylinder head bolts through fairly extreme thermal excursions during cold warmup, the root of the peculiar cylinder head retorque schedule mentioned in the service manual. This is described below in some detail.
Once the engine has warmed enough to open the thermostat the cooling system works fine. The problem is during cold-engine startup; under certain common-enough conditions genuine harm can result.
You can skip over this section if you are simply looking for the fixes.
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. My testing and measurement has convinced me that this is 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 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 to 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 now open the temperature stabilizes normally. However this is preceded by cylinder head severe overheat, followed by overcooling, and it is this temperature cycling that causes the head to grown in length (hot) then shrink (cold) in tens of seconds. Metals expand with temperature.
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!
AMC eventually recognized this problem and modified coolant flow to accommodate this lack-of-thermal-signal issue in the last two years of production.
This change, visible in these photos of a 1965 motor, causes a moderate amount of coolant to circulate between block and head at all times, critically during warm-up when the thermostat is closed. This is head-to-block circulation, bypassing the radiator.
(The "new" 199/232 motor introduced in 1964 eliminated this problem by putting the thermostat less than one inch from #1 cylinder's combustion chamber.)
As serious as the cold-startup problem is, the fix is very 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).
This fix may not be available to you, as it requires the existence of the tapped hole under the thermostat pod, in the head casting. Though that tapped hole is part of the fact 1964 engineering upgrade, I had an older engine that had the hole, blocked with a threaded plug.
This fix is a substantial improvement, like the 1964 engineering change it causes coolant to circulate through block and head at all times the engine is running. This does a number of good things at once: heat of cumbustion, mainly produced in the head over each combustion chamber, is circulated throughout the block, preventing hot spots and ensuring even warmup, something that modern engines all have. Circulation ensures that the thermostat sees the heat signal and helps purge air.
Here is an engine with a bypass hose and tee. The water pump draws/sucks from both hoses, the impeller (in the block) pushes water into the block.
When the engine is cold, the thermostat is closed, and nearly zero coolant flows in the big radiator hose up from the bottom, but the pump pulls coolant from the bypass hose, circulating between block and head.
This engine 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 far exceeds the design goals for this engine, resulting in oil overheting, which results in thin oil film, oil film failure, and in the worst case, destruction of metal parts.
Engine oil cooling is a harder problem to solve, and is covered in the lubrication section.
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.