The counterbore is slightly larger than the O.D. of the matching pipe, allowing the pipe to be inserted into the flange similar to a slip-on flange. The diamter of the smaller bore is the same as the I.D. of the matchine pipe A restriction is built into the bottom of the bore which sets as a shoulder for the pipe to rest on. This is eliminates any restriction in flow when using a socket welding flange. Specifications: Socket Welding Flanges Size: 1/2" to 60", DN 12mm to DN 1500mm view... Pressure class: Class 150 to Class 2,500, PN 2.5 to PN 250 Facing: RF / RTJ Material:view... Cabon steel: A105, A350 LF2 Stainless steel: A182 F304/F304L/F316/F316L/F321 Alloy steel: F11, F12 Packing: plywood case and plywood pallet Uses of socket weld flanges: They are ideal for small diameter applications. They are used for high pressure applications. Socket weld flanges have an internal recess on the inside diameter which allow for a smoot flow of the process fluid. They create a smooth bore with the proper welding and grinding. Note: Flange dimensions to ANSI B16.5. Dimensions "B" & "D" includes raised face thickness. Bore (SCH) "C" purchasess to Specity. Larger outlet sizes are avilable on applications. ANSI 150# / 300# 2mm Raised Face. ANSI 600# / 2500# 7mm Raised Face. Standards, Dimensions & Weight ASME/ANSI B16.5 Class 150 SW ASME/ANSI B16.5 Class 300 SW ASME/ANSI B16.5 Class 600 SW ASME/ANSI B16.5 Class 1500 SW