195.6 OHV cooling system

11-06-2025

The cooling system for the 195.6 OHV is adequate and of conventional design. Prior to 1957 the water pump was driven from the rear of the generator and mounted on the right side of the block, pumping water into the center. The block casting was changed in 1957 to mount the water pump on the front.

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 Tom’s 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.

Tom’s 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. Tom’s cars achieve this routinely with no special equipment or modifications. The entire cooling system has to be in top condition to achieve this. In a totally stock car, even with a tip-top cooling system, it’s not uncommon to hit temps over 200 degrees on hot days and/or under heavy loads. Technically, as long as the radiator isn’t boiling over it’s cooling, and you haven’t overheated until it starts boiling over. That doesn’t mean the engine should run at those kind of temps for very long! The top of the “normal” range of the factory gauge is 240 F, but that is dangerously close to boiling over (with a 15 psi radiator cap it will boil over about 250F!).  Frank ran a stock and hot rodded 195.6 for 14 years, three in the southern Idaho desert, and drove the car literally cross country. Tom has also run his in desert areas. Even under load in hot areas you really shouldn’t exceed 200-210 F. These things are old — they won’t tolerate as much heat as they did when new!

AMC specifies a thermostat temperature of 190 F or 195 F, flat-head 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 lower the temperature as you’d think — and no thermostat at all is just foolish. Physics is physics.

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 primarily reliability increasing, and secondarily performance increasing, in that you can run the engine at moderate and high sustained loads (eg. freeway driving) without overheating.

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. A stock radiator in good condition should be fine — when was the last time it was professionally cleaned? If it hasn’t been “rodded” or recored in the last 10 years you might want to try to find a radiator shop that can do that type of work — good luck doing so, they have been dying out over the last 20 years. If nothing else ditch that little four bladed fan for a five blade 14″ Flex-A-Lite or similar nylon (plastic) fan. That alone will help a lot without being very visually noticeable that it’s not stock.

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 RPM, 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, and 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.

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 could be messing with the cooling system.

My 1960 American, a very tall car, easily has 8 – 9 inches of room underneath (it can drive over parking lot curbs) and 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 achieved 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 Fahrenheit. Lower temperature thermostats worsen cooling system problems by lowering the temperature difference, lowering the radiator’s ability to shed heat.

Coolant pump

The coolant pump is nicely ordinary and reliable. New ones are not available, and in the 2020’s parts-store rebuilders sell junk. Some of the AMC and antique car vendors keep pumps in stock. It’s best to send your pump, or a spare (hard to find part that periodically will fail – best to keep a spare!), to a reputable rebuilder such as Arthur Gould Rebuilders in Massachusetts. An internet search will reveal other reputable rebuilders serving the old car hobby.

This is another component that AMC made seemingly arbitrary changes to. There’s various part numbers, I can’t tell what most of the differences are. If you shop for used ones the things to watch for are:

• • The bolt pattern. The pump you want takes five bolts, the aluminum engine pump uses four. E-Bay is saturated with aluminum-engine pumps, new and cheap. That’s because the aluminum engine was a disaster, almost none survived to even the 1970s, so spare parts were never used. The few that are still around and running were well taken care of their entire lives.
  • The large inlet’s angle relative to the block. I don’t have a known pattern for you here, but there are variations (car chassis dependent) to watch out for. A few or dozen degrees difference would be fine.
  • One or two 1/2″ NPT inlets in the top. All pumps have one for the heater hose connection. The 64 – 65 engine has two, one pointing straight up to the thermostat pod in the head casting, which has a matching 1/2″ NPT port to accommodate a short hose between pump and head — this is the engineering fix to recirculate coolant and prevent cold-start problems described elsewhere. Many aftermarket replacement pumps have both, because the later engine’s coolant pump fits and works perfectly with a plug in the extra hole. The “extra” hole can’t just be drilled and tapped in an older pump – the pump casting is slightly different (as seen in the photos below).  The hose from the bottom of the thermostat housing can be re-routed to a T in a heater hose — that will be discussed later.
  • If you buy a used (E-Bay, etc.) pump and intend to have it rebuilt, you do not need to worry about base-to-fan height! You can specify to the rebuilder what height you want. That’s it though, just base to flange-flat height. Turns out that replacement bearing and shaft assemblies come in standard sizes, and the fan flange can be pressed to pretty much any height. This hugely improves the availability of pumps. The pump below was about 5.5″ tall when I found it, and had Arthur Gould Rebuilders make it 5″ height. The longer shaft pumps seem to be more prevalent — they were intended for the big cars to move the fan closer to the radiator. I would be very hesitant to attempt to press the flange down myself. One false move and the impeller will be shattered, and there are no replacements except from another pump. The shaft would have to be supported as well as the pump body, to prevent it from rocking to one side.
  • For the record — since my main focus is on keeping these engines running and on the road, and not originality — pump shaft length varies with car chassis and engine options (A/C, heavy duty, alternators, power steering, daily weather). These pump variations all have their own part number. You can refer to the parts books found in the “Documentation and Manuals” section of this site, but the numbers are probably only good for NOS and maybe NORS parts. Those are still old, and old seals won’t last long! Have it rebuilt. AMC specific vendors may have a better idea of exactly what pump you need.

In lieu of an authoritative list of part numbers (maybe later) here’s too many photos of a pump Tom had rebuilt in 2023 for his in-car spare. I had the input shaft shortened and fan flange set to the height I needed.

I’m researching what it takes to rebuild water pumps myself. The bearing and shaft is (as of 2023) easy to get, but the seal seems scarce. A spare known good coolant pump is one of the spares I carry routinely.

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 not hard to do.

I’ve given up on brass radiators; they are twice as much money and a whole lot less cooling capacity, like 50% less in many situations. They are terrible at removing heat. I buy the largest radiator that will fit, currently the very-inexpensive oven-welded aluminum radiators from E-Bay. I’ve never experienced the horror stories of aluminum radiators recounted by many car folk; see the Coolant section. The E-Bay radiators, from various vendors, are claimed “universal AMC”, usually have “407” in the part number, such as CC407, and indeed fit most AMC cars. Tom’s 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 other possibility is to deepen the bend in the front crossmember to make room for the hose. This easily done by heating cherry red then hammering — with the engine out. Seriously it’s not hard to remove the crossmember from the car to do this. Frank has found that almost all AMC passenger cars (not counting Jeeps here!) use the same basic radiator core size — roughly 24″ wide by 16-17″ tall (not counting tanks or mounting flanges). This Champion aluminum radiator will fit 90% of AMC cars with just altering the placement of the mounting holes in the flange (note they mention another part number with the brackets on the other side of the radiator). A three row is a bit overkill for a small six cylinder engine, but the thermostat will regulate flow — it will open and close more often if the radiator sheds heat faster.

The “407” radiators have very large cores, and three or four rows of tubes. Under no circumstance does coolant temperature in Tom’s cars rise over 200F (with 195 F thermostat), and his usage includes multi-thousand-mile road trips in summer in deserts and over 5000 – 7000 foot mountains. Can you have too much radiator?

Cooling fan

The factory steel cooling fan is small. It doesn’t move much air, especially at low rpm – the main reason it’s not adequate for a “parade car” or a lot of stop and go traffic. It is also heavy, unbalanced, and the wobble probably wears the expensive water pump’s bearings out prematurely.

I run nylon plastic fans, 14″ Flex-A-Lite or Summit Racing brand. Speedway Motors is also a good source – they sell a Maradyne brand 14″ nylon fan. They are inexpensive (under $50), 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.

Some installations seem to need a 1/4″ spacer, or to clip the corners off the blades with shears. Once done they last forever. That’s a 14″ nylon fan in the photo above.

Coolant and Overflow

Mineral content in tap water and even purified bottled water, perfectly drinkable and sometimes added for flavor, is electrically active ions and the medium for electrolytic corrosion in cooling systems. Use only distilled or de-ionized water in cooling systems. This is the key to longevity. Tap or purified is better than no water, and many have used it for years. It works, but coolant doesn’t last as long and there is that electrolytic corrosion issue…

Pre-mixed coolant is pre-mixed with deionized water.  Using this, and actually correct cleaning and flushing, eliminates all coolant related problems.  You’re really not saving much money by buying concentrated coolant and distilled or de-ionized water and mixing yourself. The Dexcool in Tom’s roadster, with its iron engine and aluminum radiator, is now (2023) seven 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 in November 2023.

When the radiator needs cleaning and flushing (Tom breaks in new engines on hose water, it’s only in there for a few hours) 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 most, if not all, 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, 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 of 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 unnecessary and environmentally bad. Animals can be drawn to the sweet taste and smell and be poisoned by drinking it – which can lead to death.

Design flaw #1: Thermostat Location

The pre-1964 195.6 OHV engine has a design 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 intervention. People don’t drive today, even in their old cars being careful, as they did back in the mid 50s when this engine was designed.

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 (expansion and contraction) during cold warmup, the root of the peculiar cylinder head re-torque 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/re-torque schedule, and with the engine hot. The alleged reason is bolt torque. Tom’s testing and measurement has convinced us 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 head gasket 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. 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. This is why the temp gauge, who’s sender is near the thermostat, can be seen to fluctuate.

With the thermostat now open the temperature stabilizes normally. However, this is preceded by cylinder head severe overheat, followed by over cooling, 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.

Tom measured coolant temperatures of over 250F, accompanied by audible steam hammering, at the same time the block remained cool to the touch. He estimates during this time that there is a 150F degree temperature difference between block and head. Assuming 150F difference, he 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/contraction 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. This back and forth motion is believed to apply 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 late 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 slightly. 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). The biggest disparity is in maintenance though. Maintain head bolt torque! Frank torqued his every other year when he was daily driving, putting an average of 5,000 miles a year on the car (one or two years as much as 7,000). Torque the head bolts every 8-10,000 miles, and adjust the valves while at it!

Better fix: bypass hose and “tee”

This fix may not be so easy, 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 1964 engineering upgrade, some older engines that hole, blocked with a threaded plug. Whether it was factory or a later head swapped on is anyone’s guess on a 60+ year old engine. These days I manually drill a hole in the bottom of the thermostat pod and tap it for 3/8″ NPT and add a 1/2″ 90-degree hose barb. From there the hose runs to a T in the heater hose that connects to the water pump.

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 -65 engine) do not drill the thermostat as in the first fix, above.

Below is the 1965 engine, with 1962 cylinder head, modified for re-circulation, installed in Tom’s 1960 Rambler American wagon.

Here is a friend’s 1961 American, with a modified unknown cylinder head.

All-electronic cooling system

My roadster has been the test bed 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.