Thermosiphon cooling was generally used starting with Model T unit number 2,448.
The earliest Model T engines circulated coolant with water pumps, but Ford quickly replaced that arrangement with thermosiphon cooling. The change was part of the car’s broader engineering philosophy: remove unnecessary complexity wherever a simpler solution could provide dependable service at lower cost.
Thermosiphon action uses basic physics rather than a pump. Heated water becomes less dense and rises through the engine and into the radiator. After releasing heat, the water becomes denser and falls through the radiator, returning to the engine. This continuous circulation could cool the modest-output Model T engine during normal use.
The word “generally” matters. Unit 2,448 marks the normal transition point, but the historical description notes a few exceptions among engines produced before approximately unit 2,500. Therefore, the answer is a production-unit number rather than an absolute claim that every engine before it or after it used one system without exception.
The thermosiphon design was inexpensive and reliable, but it was not invulnerable. Heavy work, low coolant levels, or poor airflow could cause overheating, which is why aftermarket water pumps later became available for Model T owners seeking additional circulation.