EFI ST-1225 21-1/4" Flat Insert Glass Two Section Gauge with Stainless Steel Chamber Valve Center Chart: For Flat Gauges The center-to-center dimensions indicated in the charts are the minimum distances obtainable between valve centers for the corresponding gauge and valves. Charts apply to standard Rigid gauge connection 1/2" NPT Female. For optional 3/4" connection, consult EFI for corresponding valve center dimensions. Note: For Steel Valves that incorporate Threaded Union Gauge Connections (Models. EFI 323, 324, 325, 326, 423, 424, 425, 426) in either the standard 1/2" or optional 3/4" NPT Female, consult EFI for corresponding valve center dimensions.