Poly 318 Cooling System, Thermostat,

& Overheating

(applicable to poly 277, 301, 303, 313, 318, 326 and LA 273, 318, 340, 360)

The Cooling System

There can be confusion about an A-block’s cooling system and proper thermostat selection, from people telling others to not run a thermostat on performance engines to people recommending a 160° thermostat with an engine prone to running hot or overheating. To address these misconceptions, I’ll briefly summarize the A-block cooling system and use some examples.

The thermostat’s main functions are to allow the engine’s temperature to rise quickly after cold start and to then help maintain a stable running temperature within an approximate 30° range of 175° – 205°. When a cold engine starts, the water pump pushes coolant through the block, heads, intake manifold, and heater core (if installed). On a cold engine, the coolant runs into the closed thermostat valve as it tries to exit the intake manifold and is bypassed back into the water pump to cycle back through the engine. The coolant in the engine thus builds temperature quicker than it would if it entered the radiator, which helps with both cold engine performance and cabin heating. Using a 180° thermostat as an example, once the coolant reaches around 175°, the thermostat valve begins opening gradually to allow some coolant into the radiator while some is still bypassed into the pump to continue building heat. As the hot coolant passes down through the radiator, the air sucked across radiator fins/tubes and through the shroud cools the coolant before it is pumped back through the engine where it heats up again. By 200°, the thermostat is wide open flowing as much coolant as possible. Stock A-blocks are designed to perform best between 180° and 200°, and our example 180° thermostat will decrease and increase its opening/flow to help regulate the temperature so that when the engine begins heating up too much idling at a stop light or in traffic, more coolant flows through the radiator; when the engine begins cooling off too much on the open road, less coolant flows through the radiator in order to build heat in the engine.

“Normal operating temperature” depends on multiple variables including the engine, ambient temperature, driving conditions, and elevation. For example, an overbored A390 with 10.5:1 compression will want to run hotter in traffic than a stock A318 at the same ambient temperature since the 390 combustion dynamics produce more power and therefor more heat. Similarly, a stock A318 in traffic at 65° ambient will want to run cooler than the same engine in traffic at 100° ambient since the colder ambient air is more efficient at cooling the system. Similarly still, an A318 at sea level in traffic at 85° ambient will want to run cooler than the same engine in traffic at 85° ambient at 5,000 feet elevation since the thicker air is more efficient at cooling the system. In these ways, one A-block may have a normal operating range of 175° – 195° while another 175° – 205°. An ideal healthy cooling system on an iron-head A/LA should keep the engine between 175° and 205° under normal driving conditions. Depending on engine, driving, and climate variables, the engine may climb to 210° without causing concern in heavy traffic or pulling grades/towing but should not stay there constantly. 215° should draw careful focus on the temperature gauge for further increases since the engine is now overheating, and 225° should be the time to pull over and shut off the engine to avoid damage. 

Now that I’ve given a summary of the system and “normal” operating temperatures, hopefully it is clear that the thermostat has absolutely nothing to do with the minimum or maximum temperature at which the engine will run. If an engine’s cooling system is allowing it to reach 215° in normal street traffic in 80° ambient temperature, there is an issue with the system (fan/shroud/airflow, coolant level, stuck-closed thermostat, radiator flow, water pump flow, clog/air pocket in system, ultra-lean mixture, overly advanced timing). Running a 160° thermostat in an attempt to limit this overheating will not work. Upon cold start, a small amount of coolant will begin flowing into the radiator at around 155°. By around 180°, the thermostat will be completely open. If the engine is prone to running at 215°, the engine temperature will simply continue climbing to that point. In a similar way, running no thermostat will simply mean the engine will be forced to run with poor cold performance and cabin heating for a longer period until it reaches running temperature, and it will be prone to larger temperature swings in different driving conditions (idle vs. open road) since there is no valve to help smooth out the variations. In the example of this overheating 215° engine, the engine will eventually reach 215° regardless if there is no thermostat or of a thermostat’s temperature rating.

Another myth is that running a 160° or no thermostat will keep the engine colder and produce more power. There are a few issues with this claim, most of which I just explained regarding the thermostat having no impact on the minimum or maximum running temperatures. The other issue is that A-blocks, as with other internal-combustion engines, actually produce more power the hotter they are, not the colder they are. An A-block will produce more power at 205° than at 160°, proven many times over on the dyno and at the track. The myth likely stems from the fact that a colder fuel and air mixture has a denser molecular structure and packs in more power potential by volume than warmer fuel and air mixture, but the same physics don’t apply to overall engine temperature.

Thermostat Selection

So how do we select a thermostat for our A-block? In most circumstances, engines usually running in climates above 55°F will benefit from a 180° thermostat; engines usually running in climates below 54°F will benefit from a 195° thermostat that pushes the normal maintained temperature up to assist with the engine building temperature quicker at startup and quicker cabin heat since the coolant heating has to overcome the cooling effects of the ambient temperature. In this way, for example, the engine running in 30° ambient may run at 175° with a 195° thermostat rather than at 160° with a 180° thermostat since the coolant in the radiator is super-cooled. For most stock engines with a cooling systems functioning properly, a standard-flow thermostat and water pump are fine. For engines normally running in climates above 90°, engines that commonly see heavier load such as pulling grades or towing, and performance engines making over 350 HP, a high-flow thermostat, high-flow water pump, and larger 4-row-equivalent radiator will be beneficial. Thermostats are readily available through Summit Racing, JEGS, Speedway Motors, Rock Auto, Napa Auto Parts, and other retailers.

Radiator Sizing

Before the advent of mass-produced aftermarket aluminum radiators, a four-row copper-brass radiator was the best choice for performance A-blocks, especially those that would see traffic or sitting in line at the track in hotter climates. Stock or mild engines and those in cooler climates should cool fine with a good two-row or three-row copper-brass radiator. Now that aftermarket aluminum radiators are affordable and can be had in many different sizes and already setup for many makes/models/years, a good two-row aluminum radiator (4-row or more equivalent) is a great choice since aluminum is stronger than copper-brass radiators, lighter, and the tubes are larger providing superior tubing-to-fin contact area.

  • Note: Proper fan and shrouding are just as important as the radiator. The factory A-block fan will pull plenty of air and keep the vintage look, so long as it has a proper shroud to direct the air. Before upgrading to a different mechanical fan or installing an electric fan, try a properly sized shroud.

Troubleshooting Overheating

If the engine runs above 200° under normal non-traffic conditions, especially when the ambient temperature is under 85°, the cooling system and vitals need inspecting and addressing including temperature gauge/sending unit, airflow (fan/shroud/radiator fins), coolant level, radiator flow (clogged tubes), water pump flow, clog/air pocket in the system, heat riser valve, ultra-lean fuel mixture (see my carburetor tuning article), and overly advanced timing (see my article on timing an engine). If I suspect a cooling problem on a vehicle, the very first step I do is confirm that the temperature gauge is reading correctly, which I do by shooting a reliable infrared temperature gun into the thermostat housing. If the temperature there reads within the acceptable range of 175° and 200°, yet the instrument gauge shows higher, then the gauge or sending unit is the issue. Many times, an overheating problem that did not begin overnight is due to inadequate air movement through the radiator often due to a poorly designed/installed shroud. If the issue happened overnight and the pump and fan are working properly, low coolant (which means there is an issue leading to loss of coolant), a stuck-closed thermostat, or clog in the radiator or system are likely culprits. If the issue started after working on the cooling system or engine fueling and ignition, an upside-down thermostat, air pocket, ultra-lean fuel mixture, or extremely advanced timing may be the cause. A factory exhaust manifold heat riser valve stuck closed can also overheat that cylinder head and lead to higher coolant temperatures.

Leaks and Using Stop-leak Products

Put bluntly, never use a stop-leak product unless you are stranded in the middle of nowhere with a threat to your health or safety. A leaking radiator or heater core should be repaired by a shop or replaced; a leaking/corroded-through freeze plug or timing cover should be replaced; a leaking gasket (intake, head, timing cover) should be replaced. Figures 1 – 4 below demonstrate what happens when people use stop leak. This engine is a 1963 A318 block and water pump that I disassembled. It would have overheated badly and required removal, disassembly, and hot-tanking to clear the water jackets, which is far more expensive and time consuming that repairing any leak in the first place.


Poly A Block Water Pump Clogged with Stop Leak
Figure 1: Poly A Block Water Pump Clogged with Stop Leak

Poly A Block Timing Cover Clogged with Stop Leak
Figure 2: Poly A Block Timing Cover Clogged with Stop Leak

Poly A Block Timing Cover Clogged with Stop Leak
Figure 3: Poly A Block Timing Cover Clogged with Stop Leak

Poly A Block Water Jacket Clogged with Stop Leak
Figure 4: Poly A Block Water Jacket Clogged with Stop Leak