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195.6 OHV cooling system

updated 11 nov 2021

The cooling system for the 195.6 OHV is adequate and of conventional design (1958 the water pump is driven fron the rear of the generator).

This engine is very sensitive to overheating. Under no circumstances allow this engine to overheat. AMC was pretty casual about defining "overheat", the TSM considers temperatures up to 240 F or so as "normal" but in my experience with this engine, inability to hold the coolant to "thermostat" temperature plus or minus 10 degrees indicates a problem. It's just not hard to make it right.

My definition of "overheat" is more than 5 degrees over thermostat temperature. In otherwords, out of regulation. The cooling system is designed to regulate to a constant temperature. Engine (carburetor) state of tune requires a constant temperature to stay in tune. My cars acheive this routinely with no special equipment or modifications.

AMC specifies a thermostat temperature of 190 F or 195 F, flathead or OHV, all years. The engine operates best -- and the cooling system regulates best -- at this temperature. There is no scientific reason to run a 160 F thermostat -- it won't do what you think -- and no thermostat at all is just foolish.

If you run this engine hard, sustained operation at/over 3000 rpm, oil cooling becomes an issue. The relatively huge crankshaft journals do a good job of heating the oil at high speeds. Please refer to the lubrication section for discussion.

Problems and solutions

Most cooling system problems on this engine stem from advanced age and lack of maintenance, accumulated during it's lifetime. Additionally, radiators were barely adequate, cooling fans comically ineffective, and for whatever reason there seems to be great resistance in old-car culture to springing the bucks for a decent radiator.

The problem fixes on this page are reliability increasing. And secondarily performance increasing, in that you can run the engine at moderate and high sustained loads (eg. freeway driving) without overheat.

The stock type two-row brass radiator is probably not adequate for modern use. It is definitely not adequate for "parade" use, extended low-speed driving -- the useless fan is useless, and moves almost no air. A radiator is only as good as the airflow through it. This is physics and not amenable to arguments about past practices.

In 1964 AMC incorporated changes to pump and head design that solves the thermostat-placement problem. But given how much engine and parts-swapping was done in these engine's long 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.

Basic cooling system operation

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.

Lol, I love these drawings that show airflow as "in" to the radiator, but never show the "out" portion. The assumption that it somehow flows out from under the car is wrong; more specifically, it used to be true, but no longer. There's a big blob of sticky air under modern/lower cars, and if you have lowered yours, it is messing with the cooling system.

My 1960 American, a very tall car, easily 8 - 9 inches of room underneath (it can drive over parking lot curbs), cools just fine; radiator outlet temperature is 40 to 60 degrees lower than the inlet. My roadster, a highly modified 1961 American, would do 40 degree drop, average-best, and it's got six inches of clearance. When I did explicit air flow modifications which included a few square feet of ventilation to the hood, I get 80 to 100 degree temperature drop from the same cross-flow radiator, no other change. Under ideal conditions it has acheived 120 F temperature drop, top to bottom. No one ever talks about air flow!

Many thanks to David Tracy for the discussions of cooling system design.

engine operating temperature

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.


Radiator and fan

If you are sticking to a stock or restoration system, be warned that you will have to modify your behavior and expectations to drive the car. With a good clean stock radiator and stock fan, it will run hot in stop-and-go traffic and climbing long hills, but will cool adequately otherwise. If this is the case jump ahead to the Coolant section.

All of my cars are drivers or daily-drivers. I expect and get modern levels of reliability. Today this is easy to do.

I've given up on brass radiators; they are twice as much money and a whole lot less cooling capacity. I buy the largest radiator that will fit, currently the very-inexpensive oven-welded aluminum radiators from eBay. I've never experienced the horror stories of aluminum radiators recounted by many car folk; see the Coolant section. The eBay radiators, from various venders, are claimed "universal AMC", usually have "407" in the part number, and indeed fit most AMCs. (My 1960 American required re-drilling the four mounting holes to raise the radiator by 3/8" to clear the front motor cross-member; too high and it hits the hood.)

The "407" radiators have very large cores, and three or four rows of tubes. Under no circumstance does coolant temperature in my cars rise over 200F (with 195 F thermostat), and my usage includes multi-thousand-mile road trips in summer in deserts and over 5000 - 7000 foot mountains.

Cooling fan

The factory steel cooling fan is a joke. It moves nearly no air. It is also heavy, and unbalanced, and the wobble probably wears the expensive water pump's bearings out prematurely.

I run nylon plastic fans, 14" Flexalite or SummitRacing brand. They are inexpensive (under $25), absurdly light, and move a hurricane of air at idle. They also aggressively flatten out at speed. They move enough air that no shroud is needed. No contest here.

Coolant and overflow

I am embarrassed to admit that it was only relatively recently that I got rational with coolant. To "save money" I would buy concentrate and dilute with hose water. I topped off radiators (then later, overflow bottles) with tap water. Coolant lasted a couple years, maybe, and would be soon cloudy and darkened with rust. This is bad practice.

Mineral content in tap water, perfectly drinkable and sometimes added for flavor, are electrically active ions and the medium for electrolytic corrosion in cooling systems. Use only distilled or deionized water in cooling systems. This is the key to longevity.

I got rational when I switched to aluminum radiators, and did some research. Pre-mixed coolant is pre-mixed with deionized water; this, and actually correct cleaning and flushing, eliminates all coolant-related problems. The Dexcool in my roadster, with its' iron engine and aluminum radiator, is now five years old and still clear orange. The green stuff in my 1968 American, two years old, was similarly clear and clean when I sold it last month.

When I do need to clean and flush (I break in new engines on hose water, in case of issues I can easily drain it; it's only in there for a few hours). I use hose water and cleaner as per directions, then drain, and re-fill with distilled water. Run that engine hot, drain that, then install pre-mixed coolant. The distilled water flush eliminates more or less all of of the mineral-laden tap water.

(In a long and heated thread on a forum between folk who swore by using only hose water, and those who swore at them, that the outcome was determined by geographical region; some parts of the country have neutral, mineral-balance/free tap water, and some of us have hard mineral tap water.)

Overflow bottles

It's the 21st century, please run a functioning overflow bottle. Modern radiators and caps have a suction port and valve to allow overflow, from thermal expansion, to flow into the bottle, and upon cooling, to be drawn back into the engine. This also purges air out fo the system, assuming that the radiator cap is the highest point in the cooling system. Also coolant is expensive. And cars puking on the ground is unneccessary.

Design flaw #1: Thermostat location

The pre-1964 195.6 OHV engine has a designed-in flaw that is 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 sensing heat produced in the engine when initially cold. Under common 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.

Detailed analysis of the cold-warmup thermal shock problem

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 rapidly 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!

Cooling system evolution: early vs late

AMC eventually recognized this problem and modified coolant flow to accommodate this lack-of-thermal-signal issue in the last two years of production.

  1. An additional inlet was added to the coolant pump.
  2. An additional outlet was added to the thermostat pod in the cylinder head.
  3. A short hose connects the new outlet to the new inlet.

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.)



Cooling system fixes

Simplest fix: drilled thermostat

As serious as the cold-startup problem is, the fix is very simple: drill a 1/8" 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 thermostat's copper sensor button.

Placement isn't critical, but the hole wants to be inside the gasket and housing area, and not damage or nick the center portion (that opens; click the photo to see the slightly raised center with the spring inside it).

This small hole does slow engine warmup; there is no need for a larger hole.

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).

Better fix: bypass hose and "tee"

This fix may not be available to you, as it requires the existence of a 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. These days I manually drill a hole in the bottom of the thermostat pod and tap it for 3/8" NPT and add a 90-degree hose barb.

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 combustion, 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.

If you use the bypass method (or a 1964/1965 engine) do notdrill the thermostat as in the first fix, above.

Below is the 1965 engine, with 1962 cylinder head, modified for recirculation, installed in my 1960 Rambler American wagon.

Here is a friend's 1961 American, with a modified unknown cylinder head and circulation using an eary pump.

All-electronic cooling system

My roadster has been the testbed for a lot of experimentation on this engine. It has 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.