Ford Mustang/Cougar 1964-1973 Repair Guide




See Figures 1 through 4

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Fig. Fig. 1: Thermostat and housing

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Fig. Fig. 2: Unbolt the thermostat housing

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Fig. Fig. 3: Lift the housing up and slip the thermostat out

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Fig. Fig. 4: Carefully remove the old gasket from the housing and block

  1. Drain the radiator so that the coolant level is below the thermostat.
  3. Remove the coolant outlet housing retaining bolts. Pull the elbow away from the cylinder head or manifold sufficiently to provide access to the thermostat. Remove the thermostat and gasket.

To install:

Check the thermostat before installing it, following the thermostat test.

  1. Clean the coolant outlet housing and cylinder head or manifold gasket surfaces. Coat a new gasket with water resistant sealer. Position the gasket on the cylinder head opening. The gasket must be positioned on the cylinder head or the intake manifold, before the thermostat is installed. To prevent the incorrect installation of the thermostat, the water outlet casting on all engines contains a locking recess into which the thermostat is turned and locked. Install the thermostat with the bridge section in the outlet casting. Turn the thermostat clockwise to lock it in position on the flats cast into the outlet elbow.
  3. Position the coolant outlet elbow against the cylinder head, or the intake manifold. Install and torque the retaining bolts to specifications.
  5. Fill the cooling system with the recommended permanent antifreeze and water mixture. If equipped with a cross-flow radiator, follow the special instructions regarding checking the coolant level. Check for leaks and the proper coolant level after the engine has reached the normal operating temperatures.


It is good practice to test new thermostats before installing them in the engine.

Remove the thermostat and immerse it in boiling water. Replace the thermostat if it does not open more than 1 / 4 inch.

If the problem being investigated is insufficient heat, the thermostat should be checked for leakage. This may be done by holding the thermostat up to the lighted background. Light leakage around the thermostat valve (thermostat at room temperature) indicates that the thermostat is unacceptable and should be replaced. It is possible, on some thermostats, that a slight leakage of light at one or two locations on the perimeter of the valve may be detected. This should be considered normal.


Coolant level cannot be accurately checked while the engine is running, due to the construction of this radiator. Check the level while the engine is cold and fill only to the left side tank about 2 inches below the fill cap seat.