What Is an ASME B16.21 Full Face Gasket?
A full-face (FF) gasket covers the entire flange face — the flange bolts pass through holes in the gasket itself. It is used on flat-face (FF) flanges, typically cast-iron valve and pump flanges and low-pressure systems. ASME B16.21 Table 4.1-4 lists full-face dimensions for ASME B16.5 Class 150 flanges, NPS 1/2 through NPS 24.
Dimensions — B16.21 Table 4.1-4, Class 150 (mm / in.)
Dimensions in millimetres (inches), except hole diameter which is given in fractional inches. Table 4.1-4 also lists the flat-ring OD, used as a reference for the same sizes on RF flanges.
| NPS | Gasket ID (mm/in.) | Flat Ring OD (mm/in.) | Full Face OD (mm/in.) | Number of Bolt Holes | Hole Diameter (in.) | Bolt Circle Dia. (mm/in.) |
|---|---|---|---|---|---|---|
| 1/2 | 21 (0.84) | 48 (1.88) | 89 (3.50) | 4 | 5/8 (0.62) | 60.3 (2.38) |
| 3/4 | 27 (1.06) | 57 (2.25) | 98 (3.88) | 4 | 5/8 (0.62) | 69.9 (2.75) |
| 1 | 33 (1.31) | 67 (2.62) | 108 (4.25) | 4 | 5/8 (0.62) | 79.4 (3.12) |
| 1 1/4 | 42 (1.66) | 76 (3.00) | 117 (4.63) | 4 | 5/8 (0.62) | 88.9 (3.50) |
| 1 1/2 | 48 (1.91) | 86 (3.38) | 127 (5.00) | 4 | 5/8 (0.62) | 98.4 (3.88) |
| 2 | 60 (2.38) | 105 (4.12) | 152 (6.00) | 4 | 3/4 (0.75) | 120.7 (4.75) |
| 2 1/2 | 73 (2.88) | 124 (4.88) | 178 (7.00) | 4 | 3/4 (0.75) | 139.7 (5.50) |
| 3 | 89 (3.50) | 137 (5.38) | 191 (7.50) | 4 | 3/4 (0.75) | 152.4 (6.00) |
| 3 1/2 | 102 (4.00) | 162 (6.38) | 216 (8.50) | 8 | 3/4 (0.75) | 177.8 (7.00) |
| 4 | 114 (4.50) | 175 (6.88) | 229 (9.00) | 8 | 3/4 (0.75) | 190.5 (7.50) |
| 5 | 141 (5.56) | 197 (7.75) | 254 (10.00) | 8 | 7/8 (0.88) | 215.9 (8.50) |
| 6 | 168 (6.62) | 222 (8.75) | 279 (11.00) | 8 | 7/8 (0.88) | 241.3 (9.50) |
| 8 | 219 (8.62) | 279 (11.00) | 343 (13.50) | 8 | 7/8 (0.88) | 298.5 (11.75) |
| 10 | 273 (10.75) | 340 (13.38) | 406 (16.00) | 12 | 1 (1.00) | 362.0 (14.25) |
| 12 | 324 (12.75) | 410 (16.13) | 483 (19.00) | 12 | 1 (1.00) | 431.8 (17.00) |
| 14 | 356 (14.00) | 451 (17.75) | 533 (21.00) | 12 | 1 1/8 (1.12) | 476.3 (18.75) |
| 16 | 406 (16.00) | 514 (20.25) | 597 (23.50) | 16 | 1 1/8 (1.12) | 539.8 (21.25) |
| 18 | 457 (18.00) | 549 (21.62) | 635 (25.00) | 16 | 1 1/4 (1.25) | 577.9 (22.75) |
| 20 | 508 (20.00) | 606 (23.88) | 699 (27.50) | 20 | 1 1/4 (1.25) | 635.0 (25.00) |
| 22 | 559 (22.00) | 660 (26.00) | 750 (29.50) | 20 | 1 3/8 (1.38) | 692.2 (27.25) |
| 24 | 610 (24.00) | 718 (28.25) | 813 (32.00) | 20 | 1 3/8 (1.38) | 749.3 (29.50) |
Pressure Classes
B16.21 covers nonmetallic gaskets in Class 150 through Class 900. Full-face gaskets are most common at Class 150; above Class 900 use B16.20 metallic gaskets.
Materials
PTFE, rubber (NBR/EPDM/Viton), compressed non-asbestos fibre and flexible graphite — selected for the process medium and temperature.
Related Standards
- ASME B16.21 — Nonmetallic Flat Gaskets (Parent Standard)
- ASME B16.21 Flat Ring Gaskets
- ASME B16.5 — Pipe Flanges (NPS 1/2–24)
- ASME B16.20 — Metallic Gaskets
- Our PTFE Gasket Range
FAQ
When do I need a full face gasket?
When the flange is flat-face (FF) — the gasket must cover the whole face and be drilled to match the bolt circle. Raised-face flanges use flat-ring gaskets instead.
What NPS range does Table 4.1-4 cover?
NPS 1/2 through NPS 24 for ASME B16.5 Class 150 flanges, with gasket ID, OD, number of holes, hole size and bolt circle given in the table above.
Can a full face gasket be used on a raised face flange?
It physically fits but is not the recommended sealing arrangement; RF flanges use flat-ring gaskets inside the bolt circle so bolting compression is applied correctly.
What materials are available?
PTFE, rubber grades, compressed non-asbestos fibre and flexible graphite, in the thickness your flange rating requires.
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