195.6 OHV cooling system

updated 03 Dec 2023

The cooling system for the 195.6 OHV is adequate and of conventional design (1958 is the last year the water pump was driven from 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. 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 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.


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: send your pump, or a spare acquired for for the purpose, to a reputable rebuilder such as Arthur Gould Rebuilders in Massachusetts. There are others I just don't know them.

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: