This is an invitation for...
This is an invitation for disaster: Fan blades should never extend past the core. Due to the change in static pressure, the fan blades can't pull air toward themselves so they try to flex forward.
Simply stated, always use a thermostat. A relatively constant operating temperature is absolutely essential for proper emission control, good fuel economy, and performance. A thermostat will ensure the engine reaches operating temperature quickly (unlike restrictor washers alone), provide the restriction necessary to build water pressure in the block (which eliminates steam pockets), and unless they malfunction, tossing them in the trash will not cure an overheating problem and may actually make one worse.
Today two types of thermostats will be found, the poppet valve style and the sleeve style. Sleeve style thermostats are more expensive and harder to find but they often eliminate the cycling often seen on the temperature gauge as a poppet thermostat opens and closes repeatedly.
Radiator Caps
The radiator cap keeps the cooling system under pressure, generally somewhere between 5-15 psi. Pressure in the cooling system elevates the boiling point of the coolant approximately 3 degrees for every pound of pressure. Another benefit of a pressurized cooling system is the elimination of steam pockets in the water jackets.
This radiator was damaged...
This radiator was damaged because the fan blades were flexing forward as they moved past the core and behind the tank. Continued flexing resulted in one of the fan blades snagging the tank's edge, which bent the blade and made the radiator history.
It's hard to tell in this...
It's hard to tell in this photo, but the damage to the very expensive paintjob on this hood panel was caused by proximity of the headers.
There are some sources of...
There are some sources of heat in the engine compartment, such as the exhaust system, where reduction isn't possible so protection from it is the only option. Exhaust insulating wrap can reduce underhood temperatures dramatically and can prevent the damage shown in the previous photo.
When selecting a pressure cap always follow the radiator manufacturer's recommendations. As an example, one manufacturer of aluminum radiators recommends a cap with a minimum operating pressure of 16 pounds. While that may seem high, aluminum radiators will tolerate constant high pressure much better than sudden bursts of pressure that can be caused by a steam pocket that may occur with lower operating pressure. On the other hand, early radiators that were not designed for pressure operation often have large, un-reinforced upper tanks. Modifying them for pressure often results in the tanks flexing, which can lead to failure.
Coated headers are commonly...
Coated headers are commonly found on street rods for appearance purposes, but they reduce underhood temperatures as well.
Like many things that seem simple, there's more to a modern radiator cap than meets the eye. Along with a spring that keeps a preset amount of pressure in the cooling system, radiator caps also have a vacuum valve. When an engine is shut off, the coolant stops flowing through the radiator, however, the coolant continues to absorb heat from the heads and block and as a result the temperature in the system increases. As the coolant expands the pressure often increases to the point where the radiator cap's rating is exceeded and liquid is released through the radiator's overflow. When the system cools and the coolant contracts air (or liquid) is drawn back into the radiator through the vacuum valve.
Overflow Tank
Because cooling systems do, from time to time, burp out some fluid, overflow tanks are a wise addition. Allowing the cooling system to dump coolant on the ground, then suck air back in means that eventually the radiator will be low on coolant. An overflow tank catches any coolant released; it's then drawn back in when the system cools, keeping it full.
Coolant
A fifty-fifty mixture of water and ethylene glycol antifreeze will boil at 225 degrees in an un-pressurized system, with a radiator cap rated at 15 psi the boiling point is increase to 265 degrees, although we don't recommend operating an engine at that temperature.
Keeping your Cool
A few last comments on avoiding overheating: Ensure the engine is in good mechanical shape; check for proper ignition timing, advance curve and air/fuel ratio; don't use under-drive pulleys; and finally, don't forget to change coolant periodically, and check the level frequently.