Flange Bolt Size and Length: A Pre-Assembly Check Guide

A flange bolt size chart helps you identify the nominal bolt diameter and number of fasteners for a specified flange. Ordering the correct length takes another check: the fastener must accommodate the complete joint, including the mating components, gasket, nuts and any specified washers.

For stainless steel piping projects, this distinction matters at quotation, receiving inspection and pre-assembly. A stud can pass through both flanges and still be unsuitable because its threads, length or material do not match the approved joint specification.

Key Takeaways

• Identify the flange standard, nominal size and pressure class before selecting a chart row.

• Read bolt diameter separately from bolt-hole diameter.

• Confirm whether the joint requires studs with two nuts or headed bolts with one nut.

• Check length against the actual joint stack-up and the supplier’s length measurement convention.

• Keep dimensional checks separate from bolting material selection and tightening requirements.

Start With the Flange Specification

Before looking up fasteners, record the dimensional standard and edition required by the project, NPS or DN, pressure class or PN designation, flange type and facing. Identify both mating components: a flange-to-valve connection may have a different grip length from a connection between two pipe flanges.

For an ASME B16.5 connection, “4-inch flange” is incomplete. NPS 4 Class 150 and NPS 4 Class 300 require different bolt diameters even though both use eight fasteners. A PN-designated connection needs its own applicable reference.

When selecting a stainless steel flange, keep the dimensional specification and material grade as separate entries. The flange material alone does not identify the bolt pattern or establish the correct bolting grade.

If markings are missing or the components appear inconsistent, use the approved drawing and a dimensional inspection to establish their identity. The guide to reading flange dimensions explains the wider flange measurements needed for that task.

Flange Bolt Size Chart: Selected Class 150 and 300 Examples

The following examples show nominal fastener diameters and quantities for selected ASME B16.5 flange sizes. Quantities are for one complete mating joint, with one fastener through each aligned pair of holes.

Use these as lookup examples. Confirm the final fastener specification against the project’s governing standard, drawing and joint requirements. These rows do not specify length, material grade or tightening torque.

The examples also show why changing pressure class does not produce one predictable change in bolting. At NPS 2, the diameter stays the same while the quantity increases. At NPS 4, the quantity stays the same while the diameter increases. Check every relevant field when revising a material list.

For a stud-and-two-nut arrangement, eight studs normally require sixteen nuts. Do not double the stud quantity because the joint contains two flanges. Add project-required spares separately.

Bolt Diameter and Bolt-Hole Diameter Are Different

The nominal bolt diameter describes the fastener. The hole provides clearance around it.

For example, an ASME B16.5 NPS 2 Class 150 flange uses a nominal 5/8-inch fastener in a 3/4-inch bolt hole. Ordering a 3/4-inch stud from the hole measurement would confuse two separate dimensions.

Check the labels on drawings and inspection sheets. “Hole diameter,” “bolt circle” and “bolt diameter” refer to different features. If the specified fastener does not pass through the aligned holes, investigate the component identification and alignment before changing its diameter.

Confirm Whether You Need Studs or Headed Bolts

A stud has no integral head and is commonly installed with a nut at each end. A conventional hex-head through-bolt has an integral head and normally uses one nut. The resulting length dimensions cannot be exchanged directly.

For a conventional hex-head bolt, length is measured from the bearing surface beneath the head to the end. The head height is outside that dimension. A partially threaded bolt also needs enough correctly positioned thread for the nut to engage without reaching the thread runout or unthreaded shank.

For flange studs, establish whether the stated length is measured first-thread-to-first-thread or overall, including the end points. These conventions can produce different measurements on the same component. Show the required measurement basis on the order or an accompanying sketch.

Also confirm that the chosen form can be installed. Nearby pipework, a valve body or surrounding steelwork can limit the space available to insert a headed bolt or remove a stud.

Check Flange Stud Bolt Length Against the Joint Stack-Up

A chart length is useful only when its assumptions match the assembly. Check the facing, gasket, nut style and joint configuration associated with the reference before using its length in a purchase order.

Establish the Grip Length

For this check, grip length means the distance between the two outer flange bearing surfaces in the intended assembled condition, before adding external washers and nuts. It includes the mating components and the separation created by the specified sealing arrangement.

For two raised-face flanges, use a consistent drawing reference. Flange thickness dimensions can be stated with different relationships to the raised face; adding face heights again can double-count them.

Use the gasket’s specified compressed thickness where the length calculation requires the assembled condition. Its free thickness is not automatically the final gap. For a lap-joint assembly, valve connection or joint containing an intermediate component, check the complete section rather than assuming two identical flange thicknesses.

Account for Nuts, Washers and Thread Projection

For a fully threaded stud with two nuts, a useful geometry check is:

Required threaded span = grip length + specified washer thicknesses + both nut heights + specified usable thread projection at both ends.

This is a stack-up check, not a substitute for the governing length calculation. Keep all dimensions on the same measurement basis, then reconcile the result with the stud’s end-point convention, length tolerance and available order lengths.

Nut height matters: changing to a different nut style can change the required span. Washers also occupy length when specified. Insulating sleeves, insulating washers and other joint accessories need their own dimensional allowances.

Confirm full thread engagement and the required projection using the approved assembly procedure. A chamfered tip extending beyond a nut does not, by itself, prove that the required usable threads are present.

Allow for the Installation Method

Hydraulic tensioning can require additional stud extension for the tensioner to engage. Restricted access can also impose a practical maximum projection or limit the length that can be inserted.

Agree on these requirements before ordering. A longer stud is not automatically a better fit, and a standard reference length may need review when the assembly method changes.

Use a Pre-Assembly Fastener Check Sheet

Keep the flange drawing, fastener order and delivered parts tied to the same joint identifier. The following checks help expose differences before installation.

Where permitted by the project procedure, a trial fit can help confirm access and thread compatibility. Use the specified preparation and lubrication requirements; stop if engagement is abnormal rather than forcing the nut.

A trial fit does not establish pressure suitability or replace material verification. Likewise, a bolt-size chart cannot determine the correct tightening torque. After the components are accepted, follow the approved assembly procedure; the separate guide to flange bolt tightening introduces that next stage.

Make the RFQ Specific Enough to Check

A request such as “bolts for 4-inch stainless flanges” leaves the supplier to assume the pressure class, length, fastener form and bolting material. Include enough information to compare quotations on the same basis.

Provide the flange standard and edition, nominal size, pressure class, flange types, facing and mating-component drawing. Add the gasket specification and compressed thickness where applicable, the required bolting and nut grades, thread specification, washer requirements and installation method.

State the number of complete joints as well as the required quantity of individual fasteners and nuts. Identify whether length has already been approved or remains subject to drawing review. For custom flanges, supply the actual drilling and section dimensions instead of relying on a standard chart row.

When a revision changes a gasket, spacer, insulating kit or mating component, revisit the length check even if the flange size and pressure class remain unchanged. Keep the revised fastener schedule with the same drawing revision used for procurement.

Conclusion

Use a flange bolt size chart to establish the relevant diameter and quantity, then verify length against the complete joint and its installation requirements. Keeping those checks separate makes it easier to catch an incorrect chart row, an ambiguous length convention or an omitted accessory.

For stainless steel flange procurement, SDFL can review the flange requirements against the supplied specification or drawing. A clear joint description gives both procurement and installation teams a more useful basis for checking the final assembly.

Preparing a stainless steel flange enquiry?
Send SDFL your flange drawing and project requirements, including the standard, size, class, material, facing, mating component and quantity. Include the bolting schedule and joint accessories when available so the flange details can be reviewed alongside the intended assembly.

Frequently Asked Questions

Can Metric Bolts Replace Inch Bolts If They Fit Through the Holes?

Physical clearance alone is insufficient. Metric and inch fasteners have different nominal diameters, thread systems and specification requirements. Any substitution needs approval against the joint design, required strength and compatible nuts; a close dimensional conversion does not establish interchangeability.

Can I Use an Old Stud as the Only Length Reference?

Treat it as supporting information. The old stud may belong to a previous gasket arrangement, include extra extension or have been selected incorrectly. Record its measurement convention and compare it with the current drawing and assembly requirements.

Are Fully Threaded and Double-End Studs Interchangeable?

Check the specified form. A double-end stud must have sufficient thread length at both ends for the actual nut positions. Its unthreaded section and thread runout must also suit the grip. Matching overall diameter and length alone does not establish equivalence.

What If the Correct-Length Stud Is Unavailable?

Ask for the proposed alternative to be reviewed before accepting it. The review should cover thread engagement, projection, access, length tolerances and any tensioning requirement. Cutting or modifying a supplied stud also needs agreement on the finished end condition, coating and traceability requirements.

Does a Stainless Steel Flange Always Require Stainless Steel Studs?

The bolting material must follow the applicable design and service specification. Temperature, corrosion exposure, mechanical properties and compatibility all matter. The flange material name alone does not select the stud grade, nut grade or tightening procedure.