420motorsport. Thermostats, Thermostat Housings & Coolant Filler Necks

How to choose a thermostat

A thermostat keeps the coolant in the engine until it reaches its opening temperature and then gradually opens the flow to the radiator. From Gates, for example, the same type of thermostat is available with different opening temperatures, such as 180 °F and 195 °F. A lower opening temperature can keep the engine cooler under light load, but it does not increase the capacity of the cooling system – under heavy load, temperature typically depends on the radiator, fan and flow. Choose based on engine, usage and engine management, as an engine that runs too cold can result in poorer combustion and less cabin heat.

Thermostat housings and filler necks

A thermostat housing or water neck, for example from Edelbrock, must match the engine flange, the thermostat diameter and the hose size. Check whether the housing has a port for a temperature sender, and use a new gasket when fitting. Weld-on filler necks, for example from Vibrant with a 32 or 42 mm inside diameter, are used when hoses or radiators are being modified. The neck should typically sit at the highest point of the system so air can collect and be released, and the pressure cap must match the system pressure. The category also includes brands such as Mishimoto, Moroso, Wehrli and Granatelli Motor Sports.

Bypass parts and interaction with the rest of the cooling system

You will also find bypass parts, such as cooler bypass delete and thermostat bypass, which change how oil is routed through the oil cooler. Check which circuit and which engine or transmission the item is designed for, as this affects the oil operating temperature. If the engine runs too hot despite a new thermostat, the cause may lie in the radiator, the water pump or air in the system. Bleed thoroughly after fitting, and check for leaks once the engine has been warm.

Frequently asked questions

Which opening temperature should I choose for my thermostat?

It depends on the engine, engine management and usage. A lower opening temperature makes the thermostat open earlier, but does not provide more cooling capacity under heavy load. As a starting point, follow the engine's recommendation and only adjust for conversions.

Where should a weld-on coolant filler neck be placed?

Typically at the highest point of the cooling system, so air collects there and can be released during filling. The inside diameter must match the hose, and the neck material must be weldable to the radiator or pipe.

Should the gasket be replaced when the thermostat or thermostat housing is removed?

Yes, as a rule use a new gasket or O-ring. At the same time, check that the flange is clean and flat, and tighten to the manufacturer's specifications to avoid leaks.

Why does the engine run hot after a thermostat change?

Often it is caused by air in the system after filling. It can also be because the thermostat has been fitted the wrong way round, or because the radiator, water pump or fan is not up to the job. Bleed thoroughly and check the fitting direction.