Minimum Thread Engagement Depth Calculator (Le)

Estimate how deep a threaded rod end shank needs to engage into a tube bung or link so the shank yields in tension before the female threads strip in shear.

FASTENER STRENGTH · MIL-PRF-26139F MINIMUM ENGAGEMENT TABLE

Minimum Thread Engagement Depth (L_e)

Estimates internal thread engagement depth needed so the external shank yields in tension before the female threads strip in shear.

Thread Major Diameter (D)
[Inches]

E.g., 3/4″-16 UNF thread major diameter is 0.750 in.

Female Link Bung Material Pairing
MIL-PRF-26139F

Minimum-engagement multipliers per MIL-PRF-26139F. Lower shear strength materials require deeper engagement to prevent stripping.

Thread Size
UNF

Male (External) Thread Material

Female (Internal Thread) Bung Material

τ = ultimate shear strength, used with a simplified 50%-of-pitch-cylinder engagement approximation (see Limitation below).

Minimum Engagement per MIL-PRF-26139F

1.125″

✔
Commonly accepted minimum for preliminary design — not a guarantee against thread stripping in every configuration.

Basic Mode Reference

Standard Reference
MIL-PRF-26139F lists minimum thread engagement as 1.0×D for steel, 1.5×D for cast iron/brass/bronze, and 2.0×D for aluminum/zinc/plastic — a widely used fastener design rule of thumb, not derived from ISO 12240 or SAE J1120.

Assumptions & Limitations

Basic Mode multipliers are a rule-of-thumb, not a stress calculation for your specific thread class, pitch, or tolerance fit.
Advanced Mode’s shear area uses a simplified 50%-of-pitch-cylinder approximation, not the full ASME B1.1 Class 2A/2B thread shear formula — treat it as a cross-check, not a certified value.
Neither mode accounts for thread coating (plating buildup), galling, or repeated assembly/disassembly wear.
When Basic and Advanced results differ, use whichever is larger (more conservative) and add manufacturing margin.

STATIC REFERENCE DATA

Reference: Minimum Engagement by Thread Size (Basic Mode, MIL-PRF-26139F)

Precomputed values from the exact formula used above — useful as a quick lookup without re-entering inputs.

Thread Size (UNF)	Steel → Steel (1.0×D)	Steel → Cast Iron/Bronze (1.5×D)	Steel → Aluminum (2.0×D)
3/8″-24	0.375″	0.562″	0.750″
7/16″-20	0.438″	0.656″	0.875″
1/2″-20	0.500″	0.750″	1.000″
9/16″-18	0.562″	0.844″	1.125″
5/8″-18 (calculator default)	0.625″	0.938″	1.250″
3/4″-16	0.750″	1.125″	1.500″
7/8″-14	0.875″	1.312″	1.750″
1″-14	1.000″	1.500″	2.000″
1-1/4″-12	1.250″	1.875″	2.500″

WORKED EXAMPLE

5/8″-18 UNF Steel Shank into a 6061-T6 Aluminum Control Arm

A fabricator is welding a threaded aluminum bung to accept a 5/8″-18 UNF 4130 chromoly rod end shank on a billet 6061-T6 control arm, and needs a minimum thread depth.

Basic Mode: steel-into-aluminum multiplier is 2.0×D. Le = 2.0 × 0.625″ = 1.250″.
Advanced Mode cross-check: male 4130 Q&T (≈145 ksi UTS) tensile capacity Ft = At × UTS ≈ 0.256 in² × 145,038 psi ≈ 37,130 lbf. Required Le at 6061-T6’s ≈30 ksi shear strength (50%-engagement approximation) ≈ 1.337″.
Advanced Mode (1.337″) is more conservative than Basic Mode (1.250″) here — per the tool’s guidance, use the larger value and add manufacturing margin, so specify at least 1.35″–1.4″ of thread engagement.
This is a preliminary estimate. Aluminum threads are also sensitive to galling and repeated assembly wear — for a safety-critical control arm, confirm the final depth with SYZ Engineering.

Related reading: Control Arm Resource Hub →

Minimum Thread Engagement Depth Calculator (Le)

Minimum Thread Engagement Depth (Le)

Reference: Minimum Engagement by Thread Size (Basic Mode, MIL-PRF-26139F)

5/8″-18 UNF Steel Shank into a 6061-T6 Aluminum Control Arm

Need this verified for final design?

Minimum Thread Engagement Depth (L_e)