5/8-11 Bolt Specs: Tap Drill, Dimensions + Torque

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Reviewed by MLC Editorial Team | 16 August 2026

Verified by Rachel Mayfield, Supply Chain Analyst - July 2026

Close-up of hands using hand tools on metal in a workshop setting. — Photo by Daniel Smyth on Pexels

The 5/8-11 is a Unified National Coarse (UNC) bolt with a 0.6250-inch (15.88 mm) major diameter and 11 threads per inch. The tap drill for a standard 75% thread engagement is 17/32" (0.5312"), the free-fit clearance drill is 41/64", and the hex head takes a 15/16" wrench. The fine-thread version is 5/8-18 UNF at 18 TPI.

5/8-11 specifications table

Property	Value	Reference
Nominal major diameter	0.6250" (15.88 mm)	ASME B1.1
Threads per inch - UNC (coarse)	11 TPI	ASME B1.1
Threads per inch - UNF (fine, 5/8-18)	18 TPI	ASME B1.1
Tap drill - UNC (75% engagement)	17/32" (0.5312")	Standard tap drill chart
Tap drill - UNF (5/8-18)	37/64" (0.5781")	Standard tap drill chart
Clearance hole (free fit)	41/64"	Standard clearance chart
Hex head / wrench size (A/F)	15/16"	ASME B18.2.1
Tensile stress area	0.226 in²	ASME B1.1
Typical torque, Grade 5, dry	150 ft-lb	Published torque charts
Typical torque, Grade 8, dry	220 ft-lb	Published torque charts

Torque figures are typical mid-range values for clean, dry threads. Lubricated assemblies need roughly 25-30 percent less torque for the same preload; critical joints should always be calculated for the actual fastener, finish and lubrication, not taken from a generic chart.

5/8-11 tap drill size

Drill 17/32" (0.5312") for a 5/8-11 UNC tapped hole at 75% thread engagement - the standard for general work in steel and aluminium. In tough materials (stainless, tool steel) many machinists drop to 65-70% engagement with the next drill up to reduce tap breakage; in soft or thin material, higher engagement adds strength.

UNC or UNF - which thread?

UNC (11 TPI on this diameter) is the default for general fastening: faster assembly, more tolerant of nicks and dirt, less prone to cross-threading. UNF (18 TPI) gives finer adjustment, marginally higher tensile area and better vibration resistance, which is why it appears in automotive and instrument work. If you are matching an existing tapped hole, count the threads per inch against a rule before ordering.

Grade markings

SAE grade is marked on the bolt head: no radial lines means Grade 2 (low carbon, general purpose), three lines means Grade 5 (medium carbon, quenched and tempered - the workhorse), six lines means Grade 8 (alloy steel, highest standard strength). Metric equivalents run roughly Grade 5 to class 8.8 and Grade 8 to class 10.9, though the systems are not interchangeable in a controlled joint.

Metric near-equivalent

The closest metric size to 5/8-11 is M16. Near is not equal: threads will not interchange, and a metric bolt in an imperial hole (or vice versa) will strip under load. See MLC's metric bolt specification cards for the metric equivalents.

References

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Frequently Asked Questions

What is the tap drill size for 5/8-11?

The standard tap drill for 5/8-11 UNC at 75% thread engagement is 17/32" (0.5312"). For the fine-thread 5/8-18 UNF version, use 37/64" (0.5781").

What wrench fits a 5/8-11 bolt?

A standard 5/8-11 hex bolt head measures 15/16" across flats, so it takes a 15/16" wrench or socket (ASME B18.2.1).

How much torque for a 5/8-11 bolt?

Typical dry-assembly torque is about 150 ft-lb for Grade 5 and 220 ft-lb for Grade 8. Reduce roughly 25-30 percent if the threads are lubricated, and calculate properly for critical joints.

What is the metric equivalent of 5/8-11?

The nearest metric diameter is M16, but the threads do not interchange - never mix metric and imperial fasteners in the same hole.

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