How to Measure a Flange for a Cut Gasket

A gasket that is cut from a worn original repeats the original’s errors. Measure the flange instead — it takes a few minutes and gives a gasket that seats correctly, covers the sealing face properly and does not creep into the bore.

Decide the gasket type first

Two styles cover most plant work. A full face gasket covers the whole flange and is drilled for every bolt. A ring (inside bolt circle) gasket sits inside the bolts and seals only the raised face. Flat-faced flanges normally take full face; raised-face flanges normally take a ring. Fitting the wrong one distorts the load.

Take these dimensions

• Outside diameter of the flange, or of the raised face for a ring gasket.
• Inside diameter — match the pipe bore, not the flange opening.
• Bolt circle diameter — centre to centre across opposite bolt holes.
• Number of bolts and the bolt hole diameter.
• Flange face condition and the thickness of the gasket being replaced.

Get the inside diameter right

This is the dimension most often wrong. The gasket bore should match the pipe bore. Cut it undersize and the gasket protrudes into the flow, where it erodes and sheds material downstream. Cut it oversize and you lose sealing width exactly where the pressure acts.

Measuring the bolt circle

With an even number of bolts, measure centre to centre between two opposite holes. With an odd number, measure from the centre of one hole to the midpoint between the two opposite holes, or take the flange OD and work in from the hole edge. Getting this wrong by a few millimetres means the gasket will not drop over the studs.

Choosing the thickness

Thinner is usually better. A thick gasket creeps and relaxes more, and loses bolt load faster. Use the thinnest gasket that will still conform to the condition of the flange faces — old, pitted or slightly warped faces need more thickness to fill, clean machined faces need less.

Check the faces before you cut

Look at the sealing surfaces. Radial scoring across the face is a leak path no gasket thickness reliably fixes, and the flange needs refacing. Light circumferential tooling marks are normal and help the gasket grip.

Working out material usage

Once you have the OD and ID, the gasket sheet usage calculator works out the area and how many standard sheets your set of gaskets will take, which is what you actually order. We supply non-asbestos jointing sheet in a range of grades and thicknesses.

What to send us

Flange OD, pipe bore, bolt circle, bolt count and hole size, gasket thickness, and the service — medium, temperature and pressure. If you are deciding between jointing sheet grades, the jointing sheet comparison covers that, and bolt torque and sequence covers fitting it.