Does anyone know the thread size and pitch of the socket head bolts that fasten the seat rails to the floorboards? They appear to be 5/16" diameter but I'm not sure about the thread pitch. Maybe British Whitworth rather than Unified Fine. Any information on these bolts?
The thread depends on whether you have the original seat supports or replacements from Delta. The original parts use a 5/16"-24 (SAE or UNF) allen bolt, while Delta's use a 5/16"-18 (USS or UNC) allen bolt. For some reason captive nuts in the original thread don't seem to be available nowadays. To their credit, Delta provides suitable new bolts with their seat supports.