Concentric and spiral serrations are both machined flange face finishes used to help a gasket grip and seal. The important difference is the groove path: concentric serrations form separate circular grooves, while a spiral finish forms one continuous helical groove. Neither finish is automatically better for every joint. The correct choice depends on the governing standard, project specification, flange facing, gasket design, service conditions and inspection criteria.

For procurement, the safest approach is to specify more than “serrated finish.” State the facing type, serration pattern, required roughness range, applicable standard and edition, and the gasket that will be used. This guide explains what those details mean and how to include them in a flange RFQ.

Flange facing and flange face finish are not the same

A flange facing describes the geometry of the gasket-contact area, such as raised face (RF), flat face (FF) or ring-type joint (RTJ). A flange face finish describes the machined texture on that contact area. For example, two raised-face weld neck flanges may have the same ASME dimensions but different serration patterns or surface roughness requirements.

This distinction matters because an RFQ that states only “RF flange” identifies the facing geometry but may not fully define the gasket-contact surface. Conversely, “125–250 AARH” describes a roughness range but does not identify whether the lay is concentric, spiral or another approved pattern.

Concentric vs spiral serrated flange finish

Selection point	Concentric serrations	Spiral serrations
Groove path	Individual closed circles around the bore	One continuous groove progressing across the sealing face
Visual appearance	Separate rings with no continuous path from the bore to the outside	A helical tool path similar to a fine record groove
Machining	Requires controlled radial positioning for each ring	Commonly produced by a continuous facing cut with controlled feed
Sealing consideration	No single continuous groove directly connects the inner and outer edges	The continuous path makes groove geometry and gasket conformity important
Specification	Must still state roughness, tool profile and acceptance criteria	Must still state roughness, tool profile and acceptance criteria
Best choice	The pattern accepted by the applicable standard, gasket supplier and project specification

The comparison should not be reduced to a universal leak-performance ranking. A flange joint seals as a system: flange stiffness and alignment, gasket type, surface condition, bolt load, assembly procedure, temperature and pressure all influence the result. A correctly specified and inspected spiral finish can perform reliably, and a concentric finish does not compensate for damaged faces, an incompatible gasket or poor assembly.

Why serrations help a gasket seal

Machined serrations create a controlled surface texture that interacts with the gasket under compression. The peaks can help develop friction and resist lateral movement, while the valleys provide space into which a conformable gasket material can flow. The useful texture must be controlled: a face that is too smooth may offer insufficient grip for some gasket materials, while a face that is too rough or damaged can create leakage paths or cut a soft gasket.

Surface roughness is commonly expressed in microinches or micrometres Ra. In North American flange work, buyers may also use the term AARH. Because terminology and measurement conventions can vary, the purchase order should state the unit and the inspection method rather than relying on a number alone.

How gasket type changes the required finish

Different gaskets deform and seal differently. Spiral-wound, soft-cut, graphite-faced, jacketed and solid-metal gaskets should not be treated as interchangeable. Industry guidance commonly gives different recommended roughness ranges for different gasket constructions. The selected finish should therefore be checked against the gasket manufacturer’s current recommendation and the governing joint-assembly procedure.

• Spiral-wound and graphite-faced gaskets: often require a controlled serrated surface that provides grip without damaging the sealing layers.
• Soft sheet gaskets: can conform to a wider texture range, but thickness, material and service conditions affect the acceptable finish.
• Metal-jacketed or solid-metal gaskets: generally require a smoother and more carefully controlled contact surface than many soft gaskets.
• RTJ ring gaskets: seal in a precision-machined groove; they are not specified using the same raised-face serration logic.

Always resolve the gasket and facing together. If the flange finish and gasket data sheet conflict, do not assume that either concentric or spiral serrations will solve the mismatch.

What do 125–250 AARH and Ra mean?

Values such as 125–250 microinch are frequently associated with gasket-contact surfaces, but they are not a universal requirement for every flange and gasket combination. ASME PCC-1 guidance and gasket-manufacturer literature show that recommended finish ranges vary with gasket type. The ordered standard edition and project specification remain controlling.

Roughness also does not completely define the surface. Two flange faces can have a similar measured Ra but different groove depth, spacing, direction, tool radius or local defects. That is why a complete inspection should consider both the numeric roughness and the visible machining pattern.

How to inspect a serrated flange face

1. Confirm the facing geometry. Verify RF, FF, RTJ or another specified facing against the drawing and purchase order.
2. Identify the lay. Check whether the specified pattern is concentric or spiral and whether the machining is continuous and uniform.
3. Measure surface roughness. Use an appropriate calibrated instrument or approved comparator, with the required unit and sampling method documented.
4. Look for local damage. Radial scratches, dents, corrosion, weld spatter and handling marks can cross the sealing path even when the average roughness is within range.
5. Check dimensional features. Facing height, gasket-contact width, flatness and runout may affect assembly and should follow the ordered specification.
6. Record evidence. When required by the PO, retain inspection readings, photographs, drawing references and material identification with the shipment documents.

Common specification mistakes

Writing only “standard finish”

“Standard” can mean different things to the buyer, machine shop and gasket supplier. Name the standard and edition, roughness range, serration pattern and gasket type.

Confusing facing type with surface finish

RF, FF and RTJ identify geometry. Concentric, spiral and smooth describe the surface lay or finish. Both sets of information may be needed.

Using a roughness number without units

A bare value can be misread. State microinch Ra/AARH or micrometre Ra, and define the acceptance method where the project requires it.

Ignoring scratches across the gasket path

An average roughness reading may not reveal a radial scratch. Visual inspection remains important because a local defect can create a direct leakage path.

Assuming a finer finish is always safer

Finer is not automatically better. Some gasket materials rely on controlled texture for grip and conformity. Follow the joint design and gasket recommendation rather than polishing by default.

Flange face finish RFQ checklist

Include the following information when requesting a quotation or approving a drawing:

• flange standard and edition, such as the applicable ASME B16.5 edition;
• flange type, material grade, nominal size and pressure class;
• facing geometry: RF, FF, RTJ or project-specific;
• surface pattern: concentric serrated, spiral serrated or another specified finish;
• required roughness range and unit;
• gasket type, material, thickness and supplier recommendation;
• service fluid, design pressure and design temperature;
• inspection method, sampling plan and documentation required;
• any repair, recoating or preservation restrictions for the finished face.

SASA FLANGE can review these details for forged flange orders, including ASTM A182 F316L weld neck flanges. Final dimensions, facing and surface-finish acceptance should be agreed in the purchase order and approved drawing before production.

Frequently asked questions

What is a serrated flange finish?

It is a machined gasket-contact surface with controlled grooves. The grooves may be concentric or spiral and are specified together with a roughness range and other acceptance criteria.

Is concentric serration better than spiral serration?

Not in every application. Concentric grooves do not form one continuous path across the face, but joint performance still depends on the gasket, roughness, flange condition, alignment and bolt load. Use the pattern required by the governing specification.

What is the difference between smooth and serrated flange faces?

A smooth face has less pronounced machining texture, while a serrated face has controlled grooves intended to interact with the gasket. The correct finish depends on gasket construction and service requirements.

Can a spiral-wound gasket be used on a serrated raised face?

It is a common combination when the finish falls within the gasket supplier’s recommended range and the joint is assembled correctly. Confirm the specific gasket data sheet and project procedure.

Does ASME B16.5 define flange face finish?

ASME B16.5 covers pipe flange dimensions, tolerances, materials, ratings, marking and related requirements within its scope. The applicable edition, project specification and gasket/joint guidance should be reviewed together when defining the gasket-contact finish.

What should be shown on a flange drawing?

At minimum, show the facing geometry, finish pattern, roughness value and unit, relevant standard/edition, and any inspection note. The gasket specification should be cross-checked before drawing approval.

Specify the joint, not only the flange

The practical conclusion is simple: concentric and spiral serrations are manufacturing choices within a larger sealing system. A useful RFQ connects flange geometry, surface finish, gasket selection and inspection into one clear requirement. If you need a project-specific flange quotation, send the standard, size, class, material, facing, gasket and finish requirements through our Contact Us page.