Vernatherm - 18-1506 - Sierra

rather than replace the whole thermostat assembly, 99% of the time th eissue is the vernatherm. While they rarely go bad, the metal can corrode making the piston unable to move. how it works: looking at the picture of the unit, you have a pin at the far left, that rests on top of a piston. The piston has a limited amount of travel, maybe 1/4 inch. When cold the pin is pushed against the piston by a spring that is part of the thermostat assembly. inside the vernatherm is probably paraffin, when cold it allows the piston to collapse. When hot, the paraffin expands which pushes out the piston , which in turn pushes out the pin. When the pin is pushed all the way out the big bulbous portion of the vernatherm is not longer sealing the water ports of the thermostate assembly allowing water to enter the cylinder head. I was able to fix my old ones by pulling the pin out with some force and wet sanding the pin so it moved freely. But I put these way as spares. Every season , pull the assemblies apart and test the pin and test the vernatherms in a pot of hot water.

uses original t-stat housing