Simplex 631-033 — replacement cover for the older, large-format Simplex duct smoke housings.
Duct housing covers are destroyed rather than worn out: they come off for every airflow and sensitivity test, and the gasket face and screw bosses take the punishment. On this generation the cover is part of the sampling geometry, not a lid. Simplex instruction 574-710 has it seated against a gasket under four screws for an airtight seal, with the airflow baffle snapped into its slot first.
- Cover only — no sensor, detector, base, baffle or electronics supplied
- Service part for the earlier large-format duct housings, not the later compact clear-cover type
- "Old style" is the trade description for this part, not a store label
Simplex publishes no housing list for this service number, so match it to the housing already on the duct before ordering. Covers for the compact 4098-9755 and 4098-9756 generation are a different part.
