An element with a cup containing a thermally expanding material, a piston capable of translational movement along its axis, a rigid guide for guiding the piston, a seal for sealing in the thermally expanding material having an annular overall shape, centered on the axis and through which the piston passes axially, and which includes first and second opposite axial parts against which the guide and cup press respectively in an antagonistic manner to compress the seal around the piston, and an anti-extrusion washer mounted coaxially around the piston and axially interposed between the guide and first part of the seal. The first part of the seal around the piston can be supercompressed to make the degree of compression equal to a value strictly higher than that associated with an operational thermostatic element formed by the cup, piston, guide, seal and anti-extrusion washer assembled with one another without the supercompression.