Spiral Wound Gasket Dimensions (ASME B16.20, Class 300, NPS 1/2 to NPS 24)

ASME B16.20 Class 300 spiral wound gasket dimensions for ASME B16.5 raised-face flanges, NPS 1/2 through NPS 24: inner-ring inside diameter (d1), sealing element (winding) inside diameter (d2), sealing element (winding) outside diameter (d3) and centering (outer) ring outside diameter (d4), all in millimetres. The inner ring and winding (d1, d2, d3) are the same as the Class 150 gasket of the same NPS because the flange bore is unchanged; the centering ring outside diameter (d4) is larger because the Class 300 flange has a larger bolt circle, so the gasket is not interchangeable between classes. Values cross-checked across two independent reference tables — Wermac and piping-world agree on all four diameters for every NPS.

ASME B16.20 Class 300 spiral wound gasket dimensions (NPS 1/2 to NPS 24)

A spiral wound gasket seals between two raised-face flanges. ASME B16.20 sets its dimensions so that the gasket matches the ASME B16.5 flange it seats in. A typical Class 300 gasket has three parts: a solid metal inner ring that locates the inner edge of the winding and resists inward buckling; the spiral winding itself (alternating metal strip and filler) which does the sealing; and a solid metal centering (outer) ring that locates the gasket inside the flange bolts and acts as a compression stop. The four diameters below define those parts. The values are ASME B16.20 Class 300, cross-checked across two independent reference tables.

d1 is the inside diameter of the inner ring (the inner edge of the gasket bore); d2 is the inside diameter of the sealing element, where the spiral winding begins; d3 is the outside diameter of the sealing element, where the winding ends; d4 is the outside diameter of the centering ring, which seats the gasket against the flange bolts. The centering ring outside diameter d4 sits inside the flange bolt circle, so it follows the matching ASME B16.5 Class 300 flange and is larger than the Class 150 gasket of the same NPS. The Wermac and piping-world tables agree on all four diameters for every NPS; the inner-ring, winding inside and winding outside diameters (d1, d2, d3) are the same as the Class 150 gasket, which seats in the same flange bore.

Reading the dimensions

• Inner ring ID (d1) — the inside diameter of the solid metal inner ring. The inner ring fills the gap between the flange bore and the winding, stops the winding buckling inward, and reduces turbulence and erosion. ASME B16.20 makes the inner ring standard for Class 300 spiral wound gaskets across this size range.
• Winding ID (d2) — the inside diameter of the sealing element, the point where the spiral winding starts. This is the column commonly listed as the gasket inside diameter, and it is the same as the Class 150 gasket of the same NPS.
• Winding OD (d3) — the outside diameter of the sealing element, where the winding ends. The radial band between d2 and d3 is the part that actually seals when the joint is bolted up; it too matches the Class 150 gasket of the same NPS.
• Centering ring OD (d4) — the outside diameter of the solid metal centering (outer) ring. It centres the gasket within the flange bolt holes and acts as a compression stop so the winding is not over-compressed. Because it locates against the bolts, d4 is tied to the mating ASME B16.5 Class 300 flange and is larger than the Class 150 value at every NPS.

Gasket thickness, winding metal and filler material are not tabled here: they are selected for the service (typically a 4.5 mm nominal gasket with a 316 stainless winding and a flexible graphite or PTFE filler for Class 300) and do not change the four diameters above. Confirm the winding metal, filler, inner-ring metal and centering-ring metal against the service before ordering.

Selecting and matching

A spiral wound gasket matches a flange only when the standard, NPS, pressure class and facing all agree, so confirm all four before ordering. The dimensions above are the Class 300 gasket for an ASME B16.5 raised-face flange. The inner ring and winding (d1, d2, d3) are the same as the Class 150 gasket of the same NPS because the flange bore is the same, but the centering ring outside diameter d4 is larger because the Class 300 flange has a larger bolt circle. The gasket is not interchangeable between classes even at the same NPS. Match the gasket bore to the flange bore and the centering ring to the flange bolts, fit the full set, and compress the joint evenly so the centering ring seats and the winding is not crushed.

Dimensions are nominal ASME B16.20 Class 300 values and provided for identification and cross-reference. Confirm against the controlling standard and the specific gasket maker’s current data sheet before manufacture or procurement.

Reference compiled for identification and cross-reference. Dimensions are nominal; confirm against the controlling ASME B16.20 ASME B16.20 revision and the specific component data sheets before manufacture or procurement. All trademarks are the property of their respective owners.