Overhauling the Sylphon Bellows Thermostat
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.
Overhauling the Bellows
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.
Step 2. Remove the bronze frame from the housing and bellows by twisting the frame counterclockwise. The stem is held with a 1/4-20 thread at about 4-5 revolutions. The pressure of the spring will push it off, so anticipate it.
Step 3. Verify the bellows chamber is intact, and free from leaks. First remove the solder seal at the base of the thermostat and empty any remaining alcohol from inside the housing. Suggest using a small plastic measuring cup (Photo 4). 1.5 - 2.0 ounces may come out. Perform a leak down test using a jig of your choosing. In this example a PVC pipe fitting with a rubber seal, and a piece of scrap wood are held using a bar clamp (Photo 5). The PVC end cap is modified with a barb nipple, air hose and a valve to permit small amounts of air to be applied and sealed. A wooden dowel is placed into the threaded base to use as a gauge and a simple red mark is applied (Photo 6). Start by applying small amounts of compressed air (5-10 psi) and monitor the progress of the wood dowel. Fortunately for us, this unit passed the lead down test no further disassembly was required. Unfortunately for our readers, we will not be taking everything apart and won't be able to show you in detail how repairs might be made.
Photo 4 — The soldered seal is removed (or drilled) and any alcohol is drained into a graduated cup. Expect at least one ounce.
Photo 5 — Leak down test jig using PVC Pipe, a wooden dowel, a scrap piece of wood at the top and a bar clamp.
Photo 6 — Close up view of the leak down test wooden dowel. The red mark is the indicator on whether the bellows is not holding pressure.
Step 4. Repair of the bellows. If the unit failed the leak down test, then either repair/solder or replace the bellows portion if you're lucky enough to find one. Some nice methods to find pesky leaky areas are 1) fill the bellows unit with regular isopropyl alcohol and look for weeping on the outside edges (Photo 7). Photo 8 shows another method using a bright light. Clean the suspect area, apply a coating of solder and repeat the inspection process.
Photo 7 — Concept illustration of using isopropyl alcohol filled into the bellows to pin point the leak source .
Photo 8 — Concept illustration of another method used by the author to find leaks uses a bright light inside the bellows chamber. Turned up side down and with a dark cloth or perhaps a small sheet of Luan plywood in a dark room, the leak source should be visible.
Step 5. Reassemble the Sylphon Thermostat. Resolder the repaired bellows portion into the housing base if a repair was performed. Allow to fully cool before proceeding to the next step.
Step 6. Fill the Sylphon thermostat with two or three ounces of Methyl Alcohol (Photo 9) using a small syringe (from a hobby shop) and immerse the housing and bellows into a bath of ice water to lower the boiling point of the Methyl Alcohol (Photo 10). Solder shut using a high wattage electric soldering iron. Never try to use an open flame heat source as some alcohol may escape through the hole and may catch fire. The ice water minimizes this effect, but not entirely.
Photo 9 — Methyl Alcohol is available from scientific and chemist suppliers and small syringes are quite common in the paint or glue department at your local Hobby Shops.
Photo 10 — The housing and bellows unit is immersed in ice water to lower the temperature of the Methyl Alcohol while the housing is soldered. While this picture shows a shallow bowl, a large coffee can proved to most successful. Soldering must be performed using a heavy duty high wattage electric soldering iron. An open flame should be avoided as some alcohol may escape and catch fire.
Step 7. Reassemble the bronze frame to the housing and bellows using short 1/4-20 bolts and nuts. Include all the arms and clevis pins as if it were a fully completed unit. Lightly lubricate moving pieces with white grease.
Step 8. Fill a suitable container with water and immerse the completed unit into it. We used a large coffee can for this thermostat rebuild. Heat the container and monitor both the temperature and movement of the arms (Photo 11). Record this information and make your own plot if desired. Photo 12 is a graphical record of this units operation. Adding more alcohol (say four ounces) would have lowered the temperature where the shutters would have opened sooner. Verify the thermostat closes when cooled. Suggest performing several heat/cool cycles.
Photo 11 — The thermostat unit is tested on a stove to verify its operation. You may want to include a digital thermometer and dial caliber to record the results.
Photo 12 — Graph of temperature and arm movement for this Sylphon Bellows Thermostat. The Distance parameter is the distance between the two arms.
Step 9. Reinsert the Sylphon Bellows Thermostat into the radiator just as it was removed. Employing the long studs (Photo 13) is quite helpful, especially if working by yourself. Photo 14 is the completed installation.
Photo 13 — Just as the thermostat was removed suggest using temporary long studs to evenly seat the unit until the unit is close enough to bolt it in place.
Photo 14 — The thermostat unit is reinserted into the radiator with appropriate gasket and sealer.
- Clean the bronze frame, stem, arms and pins well and use small amounts of white grease on moving surfaces when reassembling.
- Since the radiator shell had to be removed to tackle this thermostat, suggest inspecting and lubricating the shutters and their operation. The thermostat will operate better if the shutters are easy to move.