Classic/Antique Car Repair: radiator and thurmostat, radiator coolant, overflow tanks

Question
What is the proper combination of lb. pressure for the radiator cap and thermostat for my 1966 mustang, 289 V8? Should they both be the same? I'm having a problem with a small of coolant coming out of the overflow tube next to the radiator cap. This happens after driving the car and then shutting it off for several minutes.

Answer
The radiator cap should be a 13 PSI cap.  The correct cap is sold under the STANT name at most auto parts stores, the correct number is 10229 or 10329, depending on the style you want.

The thermostat does not have anything to do with pressure (except indirectly), as it regulates the coolant temperature.  The correct thermostat for your car is a 195 degree unit, part number 13009 (also, a Stant number).

Burping out a small amount of coolant after shutting off the engine is a symptom of one of three things, in order of likelyhood:
1. The radiator cap is not holding pressure and needs to be replaced.  (since they are cheap and readily available, do this first).  When you install it, make sure the sealing surface down inside the neck of the radiator is not marred or dirty.

2. Overfilling of the radiator.  Coolant expands as it heats up; if the radiator is filled all the way to the brim, coolant must escape as the pressure build up.  This is why modern cars have coolant overflow tanks, but yours does not have this.  You should fill the radiator only to about 1 1/2 inches below the radiator cap opening.

3.  A more serious problem could exist within the cooling system, but you'd have other symptoms - such as a pronounced boiling or "percolating" sound after you shut the car off, especially on a hot day.  If you also have this symptom, and you have already replaced the radiator cap, write back to me and we'll discuss your next options.

In closing, if your heater puts out good heat in the wintertime, and the car does not overheat during summer driving, there is nothing wrong with your thermostat - don't worry about it at all. They very seldom fail, regardless of the anecdotal evidence you hear.

I hope this is helpful,

Dick