Don't Let Wrong Sizes Shut Down Your Pipeline – Dimension Guide Inside

author: Qishine Industry

2026-02-26

Dimensional accuracy decides whether an insulating gasket seals and isolates or fails on day one. Five critical dimensions must be defined before order: inner diameter, outer diameter, thickness, bolt hole diameter and bolt circle diameter. The structural rule is simple — Flat Face (FF) flanges take Type E full-face flange gaskets, Raised Face (RF) flanges take Type F ring-type gaskets.

Critical Gasket Dimensions

Every insulating gasket order stands or falls on six data points. Provide them as measured values, or send a flange drawing, which is still the most reliable method.

Inner diameter (ID) — the central bore. It must match the pipe bore so flow stays unrestricted while leaving enough sealing contact width.
Outer diameter (OD) — the outer edge. On Type E it equals the flange outer diameter; on Type F it sits slightly inside the bolt circle.
Thickness (T) — the industry standard is 1/8 inch (3.175 mm). Thickness drives compressibility, sealing and dielectric performance.
Bolt hole diameter — applies to Type E only. Holes must be machined to line up exactly with the flange bolt holes.
Bolt circle diameter (BC) — the circle through all bolt hole centres. It must match the flange bolt circle exactly, because it positions the gasket.
Flange standard and pressure class — ANSI 150#, 300#, 600# and the rest each carry their own dimensional series.

Type E and Type F Designs

Type E and Type F are structurally different designs, and the difference follows the pipe flanges they serve.

Aspect	Type E (full-face)	Type F (ring-type)
Coverage	Entire flange face	Raised sealing area only
Bolt holes	Precision-drilled, bolts pass through	None
Centering	Bolt holes fix the position	Self-centers inside the bolt circle
Flange face	Flat Face (FF)	Raised Face (RF)
Typical use	Contamination exclusion, easy alignment	RF joints, tighter sealing pressure

Table 1. Structural differences between Type E and Type F insulating gaskets.

Type E covers the whole flange face including the bolting area, so bolts pass through the gasket and lock its position. That keeps dust and moisture out of the joint and suits FF flanges. Type F is a plain ring that covers only the raised sealing area; its outer diameter hugs the bolt circle and it self-centers on RF flanges, giving more even sealing pressure.

Information the Supplier Needs

To get a gasket that fits the first time, send the supplier a complete data set.

Flange identity — standard (ANSI, API and others), pressure class, nominal pipe size and face type (FF or RF). This fixes the dimensional series.
Gasket type — state Type E or Type F. If unsure, describe the face type and let the supplier match it.
Measured dimensions — outer diameter, inner diameter, bolt hole diameter, hole count and bolt circle diameter; attach a drawing or clear photos when possible.
Service conditions — medium, temperature range, pressure and environment (buried, submerged, atmospheric). These drive material selection.
Material and certificates — name required materials (PTFE, G10, phenolic and similar) and any industry certificates such as API 6A or ASME B16.20 in advance.

Worked Example: Buried Gas Line

Take a buried natural gas line: NPS 8, ANSI Class 300 RF flanges, 2.2 MPa working pressure, -20 to 50 °C. The correct call is a Type F kit — outer diameter slightly under the bolt circle bore for a tight fit, standard 1/8 inch thickness, and a material such as G10 for low water absorption and chemical stability. On a long transmission system with thousands of flange points, one wrong dimension can shut the whole line down.