What is the Tolerance on the Stainless Steel Flange Gaps?

Get the gap wrong and the seal fails. That is the short version of why flange gap tolerance matters, and in oil and gas or petrochemical service, a failed seal is rarely just a maintenance issue.

The global stainless steel flanges market reached $1.276 billion in 2025. By 2032, that climbs to $1.819 billion at 5.2% CAGR. Every large-scale pipeline, refinery expansion, or power generation project pulling that number up depends on flanged joints that hold under pressure. Getting the tolerance right is what makes that possible.

In this guide, you will learn:

• What the acceptable tolerance for a flange gap is
• How ASME B16.5 defines dimensional tolerances
• What H6 and H7 tolerances mean in flange applications
• How ANSI flange tolerances compare to ASME
• How tolerance directly affects sealing performance
• What acceptable gap tolerance looks like in industrial piping

What is the Tolerance for a Flange Gap?

The flange gap is the space between two mating flange faces before the gasket is compressed. That gap is not arbitrary. The gasket needs enough compression to seal but not so much that it extrudes or damages the sealing surface.

Most piping engineers work to a pre-bolt gap of 3mm to 5mm for standard gasket types, with the final compressed gap determined by the gasket manufacturer’s compression specifications. Installation error is where problems originate. Misaligned flanges create uneven gaps across the bolt circle. One side compresses correctly while the other stays open. Tightening bolts harder does not fix uneven compression once it exists. ASME B16.5 allows a maximum alignment deviation of 1.5mm across the bolt hole pattern, a figure that field installations frequently exceed without anyone noticing until the joint leaks.

Flange Alignment and Gap Tolerance: Standards and Best Practices

Alignment tolerance and gap tolerance work together. A flange manufactured to exact dimensional tolerance still fails if installed with poor alignment. The two must be controlled simultaneously.

Best practice for large diameter stainless steel flanges in EPC projects includes the following controls:

1. Pre-assembly dimensional verification

Pre-assembly dimensional verification of both mating faces should be carried out before installation.

2. Bolt tightening control

Controlled bolt tightening sequences should be applied to distribute load evenly across the gasket.

3. Gap measurement

Gap measurement should be taken at multiple points around the bolt circle before final torquing.

Any point-to-point gap variation exceeding 1.5mm should be investigated before proceeding.

4. Installation warning

Forcing a misaligned joint to compress through bolt torque alone creates localised gasket stress that compromises long-term sealing integrity.

What is the Flange Tolerance as per ASME B16.5?

ASME B16.5 publishes specific dimensional tolerances for every flange type and pressure class. Key tolerance values affecting gap control and sealing performance include the following.

Raised face diameter tolerance runs to ±0.8mm for flanges up to NPS 24. Flange thickness tolerance is +3.2mm/-0mm, meaning flanges can be thicker than nominal but not thinner. Bolt hole circle diameter tolerance is ±0.5mm for most sizes, directly affecting how evenly the bolt load distributes around the joint. 

Bolt hole angular position tolerance is ±0.5 degrees from the true position. Across a large diameter flange, that half-degree translates into meaningful linear displacement affecting bolt alignment and gasket loading distribution.

Facing finish tolerance is the most critical sealing variable. ASME B16.5 specifies 125 to 250 microinches Ra for raised face flanges used with spiral wound gaskets. Surfaces outside this range will not seat the gasket correctly, regardless of gap control during installation.

What are H6 and H7 Tolerances in Flanges?

H6 and H7 are ISO tolerance designations for holes, commonly referenced in precision machined flange bore and fitting applications. The H designation indicates the hole tolerance zone, with the number indicating the tolerance grade, where lower numbers mean tighter tolerances.

H7 is the most commonly specified tolerance class for flange bore applications requiring a good fit with mating components. It allows a positive deviation from nominal bore size, meaning the bore can be at or above nominal but not below. H6 is tighter still, used where a higher precision fit is required between the flange bore and mating pipe or fitting.

In large diameter stainless steel flange applications, H7 bore tolerance is typically specified for slip-on flange configurations where the pipe must slide through the bore cleanly before welding. Maintaining H7 tolerance across large diameters like DN600 to DN4000 requires precision machining capability that not all manufacturers consistently deliver.

ANSI Flange Tolerances: How They Compare to ASME

ANSI flange tolerances and ASME B16.5 tolerances are effectively the same standard in modern practice. ASME B16.5 incorporated and superseded the older ANSI B16.5 designation. What was published as ANSI B16.5 is now published as ASME B16.5 with the same dimensional tolerance tables.

The distinction matters historically because older project documentation may reference ANSI B16.5 tolerances. Those tolerances are numerically identical to current ASME B16.5 values. A flange specified to ANSI 150 and a flange specified to ASME B16.5 Class 150 carry the same dimensional tolerance requirements.

How Tolerance Affects Flange Sealing

Sealing performance is directly determined by how closely manufactured dimensions match specified tolerances. Three tolerance variables drive sealing outcomes.

Facing finish tolerance affects gasket seating. A surface too rough prevents uniform gasket contact. Too smooth, and the gasket cannot grip the sealing face adequately under thermal cycling. Both conditions produce a leakage risk that bolt torque alone cannot correct.

Raised face diameter tolerance affects the available seating area for the gasket. A face diameter outside tolerance may position the gasket partially beyond the sealing zone, reducing effective seating width and increasing unit load on the remaining contact area.

Bolt hole position tolerance affects load distribution. Holes out of position create uneven bolt spacing that prevents uniform gasket compression around the full circumference. In high-pressure stainless steel flange applications, non-uniform compression produces preferential leakage paths at the under-compressed zones.

Acceptable Flange Gap Tolerance in Industrial Piping Systems

In industrial piping, acceptable gap tolerance varies by service condition and gasket type. For spiral wound gaskets in standard raised face applications, a uniform pre-bolt gap of 3mm to 5mm is the working target. For ring-type joint flanges used in high-pressure service, the gap tolerance is tighter because the metallic ring gasket requires more precise face-to-face spacing to seat correctly.

Leakage risk increases non-linearly with gap deviation. A gap of 1mm outside the target range creates a measurable sealing risk. A gap of 3mm outside the target in a high-pressure stainless steel application is a near-certain source of field leakage. Large diameter flanges compound this risk because a gap variation that is minor as a percentage of the diameter becomes significant as an absolute dimension at DN600 and above.

For Longan Flange’s large diameter stainless steel production from DN600 to DN4000, dimensional tolerance control across the full manufacturing process, from forging through final machining, is what ensures the gap tolerance that field installation depends on. With 160 high-precision machining centres and 150 test equipment pieces, dimensional verification happens at every stage rather than only at final inspection.

Learn more about Longan Flange’s manufacturing capabilities at their website.