By Chris Wantuck
The Sylphon Bellows Thermostat may appear complex, they are in reality so simple that overhauling them and re-using them again and again can be a good thing. Here we examine rebuilding a Sylphon Bellows Thermostat that was used extensively in the 1920's and 1930's. This type of thermostat controlled the vertical metal strips (or "shutters") that were in front of the radiator. Closing the shutters prevents air from flowing through the radiator raises the engine's coolant temperature.
When the coolant (usually an alcohol and water mixture) reaches the warm temperature, this type of thermostat, mounted at the top of the radiator, responds to the warm coolant and pushes the shutters open. The open shutters allow air to flow through the radiator, cooling the engine. The process repeats as needed, usually ending up with the shutters either being partially- to fully-open. A cross section view (Photo 1) shows how the Methyl Alcohol, when heated, produces pressure to the base of the bellows and pushes the stem upwards. The corrugated shape of the bellows pushes the stem down when the alcohol cools and pressure is reduced. Methyl Alcohol is used as it has the lowest boiling point of all of the alcohols. Photo 2 shows the three major components of the Sylphon Thermostat while Photo 3 shows an inside peek into the housing portion revealing the size and shape of the corrugated bellows and the threaded base.
Photo 1 — Cross section view of the Sylphon Bellows Thermostat. The grey colored area between the bellows wall and the outer shell illustrates the Methyl Alcohol that when heated, pushed against the base of the bellows.
Photo 2 — The three major components of the Sylphon Bellows Thermostat, left to right) the frame with stem, 2) spring, and 3) the base housing and bellows. The frame and stem are removed by unscrewing the stem from the base. Since the spring provides constant pressure, care should be taken when separating the frame (& stem) from the base housing.
Photo 3 — Inside view of the bellows portion. Note the threaded base at the bottom which holds the stem and the top of the bellows which is soldered to the housing.
Step 1. Remove the thermostat from the radiator. Remove the clevis clips and arms first and set them aside noting the orientation of each arm. Next, remove the main pin, paying close attention to the locking clip. When removing the small bolts around the flange, spring pressure may cause the bronze frame may snap forward unexpectedly. Either have a helper hold the frame until the bolts have been removed or, better, use long studs and nuts at opposite sides, which allows the unit to be slowly removed (see Photo 13 below). This will be necessary when reinstalling the thermostat, so it makes sense to use them now.