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Bolt Hole Patterns and Compatibility Between AWWA Flange Classes

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In the design and installation of AWWA flanges, pressure rating is an important factor, but equal attention must be given to the bolt pattern. The way a flange is drilled determines not only how it is fastened but also whether it is compatible with other flanges or not. If the drilling does not match, the flanges will not mate, regardless of pipe size or pressure rating. A bolt pattern is defined by the number of holes, the hole diameter, and the bolt circle diameter (BCD). These dimensions are standardized under class designations such as Class 150 or Class 300. It is also  essential to note that bolt pattern classes are different from AWWA pressure class letters (A, B, D, E, F), which are often confused with each other. In this blog post we are going to explain how bolt patterns differ between classes and why this matters for compatibility.

What Are Bolt Patterns?

A flange’s bolt pattern is defined by three main dimensions:

  1. Number of Bolt Holes: How many fasteners are used to secure the flange.

  2. Bolt Hole Diameter: The size of each hole, which dictates the bolt size.

  3. Bolt Circle Diameter (BCD): The imaginary circle that connects the centers of all bolt holes.

AWWA Flange Classes and Bolt Patterns

AWWA flanges are governed by standards C207 and C228, and they are divided into pressure classes A, B, D, E, and F. Each class follows a drilling standard that aligns with ANSI/ASME flange bolt patterns.

AWWA ClassPressure RatingBolt PatternCompatibility Notes
Class AUp to 50 psi (primarily light service)

ANSI/ASME

Class 150

Shares Class 125/150 drilling; compatible/interchangeable with B, D & E; typically used for low-pressure or open-ended irrigation/exhaust systems, and all other low-pressure specialty pipe systems.
Class BUp to 86 psi

ANSI/ASME

Class 150

Shares Class 125/150 drilling; compatible/interchangeable with D & E; often used for low-pressure waterworks
Class DUp to 175 psi (4″–12″)150 psi (14″ and larger)

ANSI/ASME

Class 150

Shares Class 125/150 drilling; compatible/interchangeable with B & E; widely used in most waterworks and many industrial applications
Class EUp to 275 psi

ANSI/ASME

Class 150

Shares Class 125/150 drilling; compatible/interchangeable with B & D; used in higher pressure tolerance systems requiring Class 150 drilling
Class FUp to 300 psi (4″–48″)

ANSI/ASME

Class 300

Class 250/300 drilling; not compatible with A, B, D, or E; requires dedicated Class 300 components for higher-pressure waterworks and many other applications

Hub vs. Ring Flanges: Do They Affect Bolt Patterns?

Within each AWWA class, you may find both hub-type flanges (weld-neck style) and ring-type flanges (slip-on or plate style). In most cases, these share the same bolt pattern for the given class. However, dimensional variations may exist depending on the manufacturer or pipe size. Always check the specification sheet for bolt count, hole diameter, and BCD before assuming interchangeability.

Other Flange Types and Compatibility Notes

1. Backing Flanges

Our backing rings and stub ends are manufactured to the same class (Class 150).

  • An AWWA Class B, D, or E flange can be machined with a chamfer to function as a backing ring alternative.

  • The modified flange remains fully compatible with stub ends.

2. Exhaust Flanges

Unlike AWWA waterworks flanges, exhaust flanges follow manufacturer-specific patterns. But many industrial exhaust applications utilize AWWA Class A, B, or D flanges.

  • For example, Caterpillar and Detroit Diesel each use their own proprietary bolt layouts.

  • Always verify the pattern against the Automotive/OEM specification.

3. DIN Flanges

European DIN 1092 flanges are not compatible with ANSI/ASME/AWWA bolt patterns.

  • Recommendation: Avoid mixing DIN with ANSI/ASME/AWWA systems.

3. ANSI/ASME Flanges

API International specializes in class 125/150 and class 300 ANSI/ASME flanges. The bolt hole patterns align if they share the same class.

  • The bolt hole patterns of ANSI/ASME flanges should be compatible with other flanges if they share the same class (class 150 bolt hole pattern aligns with other class 125/150 bolt hole patterns).

Quick Reference Table

AWWA ClassBolt PatternCompatibility Notes
A, B, D, EClass 150Fully interchangeable with each other
FClass 300Requires matching Class 300 flanges only
BackingClass 150Compatible with stub ends; can be machined from B/D/E
ExhaustManufacturer-specificManufacturer/OEM-specific: Must match OEM pattern (e.g., Caterpillar, Detroit, etc.). Compatible with A, B, C, D, E (drilling/bolt pattern and pressure must be verified)
DINDIN 1092Only compatible with DIN flanges

Why Compatibility Matters

Using mismatched bolt patterns can result in:

  • Misalignment: Bolts won’t seat properly, preventing assembly.

  • Leaks: Poor gasket compression leads to water loss or contamination.

  • Mechanical failure: Stress concentrations can damage flanges, bolts, or connected piping.

Practical Guidance for Engineers & Planners

1. Match class numbers, not just pressure ratings

“Class 150 pattern” = A, B, D, E. “Class 300 pattern” = F

2. Verify with specification sheets

Always cross-reference API International or AWWA spec tables for exact drilling dimensions before selecting flanges.

3. Stay within the same bolt pattern group

A, B, D, and E can be mixed; Class F requires its own matched components. ANSI/AWWA vs. DIN will not align.

4. Check both pressure and pattern

Even if bolt holes line up, pressure requirements must also be considered.

5. Consult manufacturers when in doubt

For unusual pipe sizes or mixed flange applications, confirm details directly with the supplier.

Final Thoughts

Bolt hole patterns are one of the main factors to consider for  flange compatibility. AWWA Classes A, B, D, and E all follow the Class 150 bolt pattern and are interchangeable. Class F uses the Class 300 bolt pattern and must be paired with matching components. Backing flanges, exhaust flanges, and DIN flanges add further variation, making specification checks essential. By focusing not only on pressure ratings but also on bolt patterns, engineers and planners can avoid costly mistakes, ensure leak-free connections, and keep waterworks systems operating safely.
Our sales team at API can answer any questions about your flange requirements. Depending on its application, they can also recommend alternative products that may be better suited for your system. We also provide custom flange solutions tailored to your project specifications. Get connected with a dedicated sales representative today! Or call us: 503.692.3800