Thread engagement is the length of contact between the screw’s external thread and the component’s internal thread. In a tapped component, the required length depends on the relative strengths of the screw and base material as well as thread geometry and load. A simple ratio of engagement length to nominal diameter, d, is a quick reference only. The table below reproduces selected Bossard recommendations for M6–M16 cut internal threads, using two material-strength groups.

Metric Thread Engagement Length Classes

Read each material and screw class together

The table separates component material strength from screw property class. For example, a recommendation for an Rm greater than 360 N/mm² material is not automatically suitable for a softer casting, aluminum alloy, or a material with local damage. Likewise, increasing the screw property class does not by itself establish that the internal thread can carry the intended preload or service load.

Bossard labels its recommendations for cut internal threads in the stated metric range. The nominal ratio is converted to length by multiplying by the screw diameter. For an M10 screw, 1.0d corresponds to 10 mm of engaged thread; 1.25d corresponds to 12.5 mm. The calculation does not account for incomplete threads at the entry, chamfers, bottom clearance, or the amount of usable full-form thread in a blind hole.

Understand bracketed fine-thread figures

Bossard distinguishes practical trial-based recommendations from bracketed theoretical values associated with VDI 2230. The fine-thread figures shown in brackets should therefore be understood as theoretical references, not as equal evidence to the non-bracketed practical values. Check the source sheet and the applicable calculation method before using a bracketed number in a design decision.

Thread pitch changes the number of engaged threads for the same axial length. The table therefore keeps coarse and fine thread recommendations distinct. Confirm the exact pitch, tolerance class, and tap drill in the drawing and manufacturing plan. Do not apply a coarse-thread ratio to a fine-thread connection or infer an unlisted size from the selected M6–M16 scope without checking the original source.

Check the joint, not only the ratio

Engagement length is only one part of a bolted-joint calculation. The design should consider preload, external axial and shear loads, joint separation, fatigue, temperature, vibration, corrosion, lubrication, and the stiffness of clamped parts. Check stripping failure in both the internal and external threads, bearing stresses under the head or nut, and the strength of the surrounding material. Thin walls, edge distances, inserts, coatings, and soft or porous substrates can govern before the nominal engagement length does.

Bossard notes that actual calculation is required for exact sizing. Use the current VDI 2230 method or another applicable engineering standard with verified material properties and the real load path. A longer thread may not provide a proportional increase in joint capacity, and excessive engagement can create assembly or tolerance problems. For critical joints, have the design reviewed by a qualified fastener engineer or supplier.

Manufacturing and inspection considerations

Verify that the tapped hole has sufficient depth for the required full-form engagement plus lead and tool clearance. Blind holes need additional depth below the engaged length. Confirm that the tap remains perpendicular, chips are controlled, and thread quality meets the drawing. For repairs, inspect the substrate and do not assume that a previously stripped hole still meets the original material assumptions.

Document the selected material group, measured or specified strength, screw class, pitch, engagement length, and calculation basis. This makes the choice auditable when the component or supplier changes. A standard nut may have a different geometry and validated capacity from a tapped part, so do not substitute a nut-based rule without checking its scope.

References

• Bossard: Length of engaged thread
• Bossard technical information, January 2025

Use the engagement ratio as a preliminary check. Verify the current recommendation, full joint calculation, thread dimensions, material condition, and manufacturing details for the actual connection.