125-250 AARH Serrated Finish Meaning

The 125-250 AARH serrated finish is the standard surface roughness specified by ASME B16.5 for raised face (RF) and flat face (FF) flanges. The numbers 125 and 250 refer to the minimum and maximum Arithmetic Average Roughness Height in microinches. This controlled roughness range, combined with concentric serrated grooves, provides optimal gasket sealing for most piping services.

125-250 AARH means matters for flange procurement, gasket selection, and joint assembly in process piping.

What the Numbers Mean

Parameter	Value	Explanation
125 AARH	125 microinches (3.2 um Ra)	Minimum roughness; smoother than this reduces gasket grip
250 AARH	250 microinches (6.3 um Ra)	Maximum roughness; rougher than this prevents full gasket seating
Serrated	Concentric circular grooves	Groove pattern machined into the flange face
Groove pitch	~45 grooves per inch (per ASME B16.5)	Spacing between consecutive grooves
Groove depth	1/64 in. (0.4 mm)	Depth of serration cuts

The 125-250 AARH range means the arithmetic average of all surface peaks and valleys, measured across the grooves, falls between 125 and 250 microinches. This is a statistical average; individual peaks may exceed 250 microinches while the overall average remains within specification.

Why 125-250 AARH Is Standard

ASME B16.5 established the 125-250 AARH range as the best compromise for the widest variety of gasket types. The reasoning:

• Below 125 AARH: The surface is too smooth. Soft gaskets (CNAF, graphite) lose their mechanical bite and may creep or extrude under bolt load and operating pressure.
• Above 250 AARH: The surface is too rough. Gasket material cannot fully fill the deep grooves, leaving spiral leak paths from the bore to the outside diameter.
• 125-250 AARH: The grooves are deep enough for the gasket to interlock with the flange face, yet shallow enough for the gasket to fully fill each groove valley.

Finish Requirements by Gasket Type

Different gaskets tolerate different surface roughness ranges. While 125-250 AARH works for most gaskets, some applications require finer finishes:

Gasket Type	Recommended AARH	Notes
Spiral wound (with graphite filler)	125-250	Standard for RF flanges
Compressed non-asbestos fiber (CNAF)	125-250	Standard for RF flanges
Flexible graphite sheet	63-125	Smoother finish recommended
PTFE envelope	63-125	Smoother finish reduces cold flow
RTJ metallic	63 max	Precision-machined groove required
Elastomeric (rubber)	250-500	Rougher finish improves grip

Pro Tip

When re-machining a damaged flange face, always specify “125-250 AARH concentric serrated” to the machine shop. A smooth lathe finish without serrations may fall within the AARH range numerically but lacks the groove pattern required for proper gasket seating. The serration pattern is as important as the roughness number itself.

Inspection and Verification

The 125-250 AARH finish is verified using a surface profilometer or by comparison against a surface roughness comparator block. For critical services, four measurements at 90-degree intervals around the flange face should all fall within the 125-250 range. The measurement stylus must traverse perpendicular to the serration grooves.

Flanges manufactured per ASME B16.5 are supplied with a standard 125-250 AARH serrated finish unless otherwise specified on the purchase order.

Read the full guide to flange face finishes