Abstract: A repeatedly but easily compressible seal structure providing high resistance to the passage of fire across a long seal path between two opposing members is provided by an assembly including a long spring member that is laterally compressible, an outer circular cover of an ablative material and an inner core of high temperature resistant material. The spring is fitted within the cover and tends to restore it to circular cross section after compression between the opposing members is released, so that the cover provides a seal for normal operation. The inner core at least substantially fills the space within the compressed spring. Thus, in the event of fire, the cover acts as an initial ablative barrier but after it decomposes on the exposed side the spring holds the core in position to maintain the thermal and fire barrier for a desired interval.