The 3850 Fire bolt is a spring-loaded bolt, mounted in the inactive leaf of a pair of doors. The bolt is held in the retracted position by a fusible membrane which melts at 450 degrees and allows the bolt to engage and lock-up the pair of doors. Designed for use in conjunction with an automatic flushbolt or self-latching flushbolt. It replaces the bottom bolt of a set of auto/semi-auto flushbolts or can be used with a vertical rod panic device. Flush bolt combinations include: 3810×3850, 3815Lx3850, 3820×3850 and 3825Lx3850.