[edwardc]

What you have there, to quote Neil, is the "Holy Grail" of thermostats, the original Holley 3-spring dual-action thermostat. Take good care of that puppy. They are no longer made, and the replacement dual-action 'stats are increasingly hard to find and also expensive. This is what prompted Don to develop the more economical single-action conversion kit.

The thermostat initially starts to open at 140 degrees. This allows cooling water to begin to flow through the block. As it heats more, it opens farther, until at 180 degrees the flat disk on the top side is completely pressed against the circular boss inside the housing, cutting off all flow through the bypass loop and forcing all coolant through the block. Thus, in normal operation, the late model engine temperature is allowed to vary between 140 and 180.