Servicing Bi-Metal Choke Springs

 How to remove and replace Bi-Metal Carburetor Choke Springs

Bi-Metal choke springs are used to convert a temperature change into mechanical displacement. The coil is made from two strips of different metals which expand at different rates as they are heated.  The different expansion rates of the two metals will cause the coil to wind or unwind if heated, and return in the opposite direction if cooled.

The bi-metal spring can be heated in several ways.  Electric chokes us a 12volt heating element or heat could come from hot air tubes routed in exhaust gases.  Divorced chokes are usually mounted on engines intake manifold and can be heated by exhaust gases or engine casting temperature.

How to remove and replace Bi-Metal Choke Springs on thermostats:

Generally, the innermost edge of the bi-metal coil is wedged between a split rod made from aluminum or brass.  If you need to reverse the rotation of the thermostat coil, you can accomplish this by removing the spring and flipping it over, effectively reversing the rotation of the coil.  

Carefully inset a thin flat bladed screwdriver under the innermost coil where there is a 90 degree bend that is positioned in the split of the aluminum or brass shaft.  Carefully twist screwdriver blade back and forth.  Most times this will be enough to move the coil end up from the split shaft allowing you to grab the 90 degree potion of coil and gently pull straight out of the split shaft.  On stubborn units try adding a small amount of penetrating oil to split shaft.  Sometimes the split shaft will need to be opened/spread slightly, but be very careful when attempting this as you don’t want to break the split shaft or make it so loose that it will no longer retain the coil.  You may be able to carefully squeeze the two side of the shaft slightly to close up the gap before you re-insert the coil.  After flipping coil and reinstalling the end of the coil in the split shaft, ensure that these is sufficient tension to retain coil properly.

Note:  Some early thermostats are made from cast material (for example, early metal Rochester thermostats) and the cast split shaft tabs can break VERY easily and are NOT repairable.