Unified National Fine · 18 TPI

5/8-18 UNF thread

Every dimension of a 5/8-18 UNF thread as the configurator applies it: major and minor diameter, pitch, the tap drill and the hole-size window by depth and class of fit, the thread-relief undercut, and the standard drills either side, with the catalog parts that carry it.

5/8-18 UNF is a Unified National Fine thread: 0.6250 in nominal major diameter and 18 threads per inch, a pitch of 0.0556 in from one crest to the next. It is a 60° V-form thread.

The major diameter, 0.6250 in, is the outside of the external thread and the diameter a shaft end is turned to before the thread is cut. The minor diameter, 0.5649 in, is the root: the thread is 0.0301 in deep per side between the two.

The chart tap drill is 9/16 (0.5625 in), which leaves about 87 percent of a full thread. The configurator does not stop at the chart value for a tapped hole: it sizes the drill from the depth of the hole and the class of fit using the window in the second table, so it selects 14.5mm (0.5709 in, 75% thread) for a hole up to a third of a diameter deep or between a third and two-thirds of a diameter deep; 37/64 (0.5781 in, 65% thread; the nearest standard drill, since none lands in that window) for a hole between two-thirds and one and a half diameters deep or deeper than one and a half diameters. The drawing callout names the drill and the engagement it is based on.

Configure a part →

5/8-18 UNF thread dimensions

Inches at the resolution the configurator holds them. 5/8-18 UNF is a 60° V-form thread. The undercut is the thread-relief groove cut ahead of a shoulder for a single-point-cut external thread, so the tool has somewhere to stop: 3 × pitch wide (DIN 76 Form A) with its root 0.0150 in below the minor diameter, which is how the configurator derives it when a thread ends in an undercut.

Download this table (CSV)

5/8-18 UNF tapped-hole limits by depth and class of fit

Minor-diameter limits of the tapped hole in inches, from the Machinery's Handbook hole-size window. D is the 0.6250 in major diameter and the depth is the length of thread engagement; the configurator assigns the band from the hole's depth (at or under 1/3 D, 2/3 D and 1.5 D, then everything deeper) and applies the last band to any hole deeper than 1.5 D. A deeper hole gets a larger minor-diameter window, so less thread engagement, and the drill shown is the largest standard drill inside that window, with the percent of thread it leaves. Where no standard drill lands in a window the nearest is shown and marked.

Download this table (CSV)

Standard drills around the 5/8-18 UNF tap drill

The two number, letter or fractional drills just under and just over the chart tap drill, with the chart drill between them. Percent of thread is (major diameter − drill diameter) ÷ (1.299 × pitch), the figure the configurator reports on the callout; a smaller drill leaves more thread and loads the tap harder. When no inch drill lands in the window the configurator searches the metric index as well, which is how a metric drill can be the pick for an inch thread in the table above.

Download this table (CSV)

Part of Thread and tap-drill reference. Other pages in the series: 1/4-20 UNC · 5/16-18 UNC · 3/8-16 UNC · 7/16-14 UNC · 1/2-13 UNC · 9/16-12 UNC · 5/8-11 UNC · 3/4-10 UNC · 7/8-9 UNC · 1-8 UNC · 1-1/8-7 UNC · 1-1/4-7 UNC · 1-3/8-6 UNC · 1-1/2-6 UNC · 1/4-28 UNF · 5/16-24 UNF · 3/8-24 UNF · 7/16-20 UNF · 1/2-20 UNF · 9/16-18 UNF · 3/4-16 UNF · 7/8-14 UNF · 1-12 UNF · M6×1 · M8×1.25 · M10×1.5 · M12×1.75 · M14×2 · M16×2 · M18×2.5 · M20×2.5 · M22×2.5 · M24×3.

Class of fit and depth of engagement for 5/8-18 UNF

Class of fit sets how tightly the mating threads engage: 1B and 2B share one hole window, 3B has a tighter one. Most work is 2B, and the configurator applies the 1B/2B window unless you choose 3B on the hole. The window then moves with depth: a deeper hole gets a larger minor diameter, so less thread engagement, because thread strength stops improving above roughly three-quarters engagement while every extra bit of material loads the tap.

D in the table is the 0.6250 in major diameter and the depth is the length of thread engagement. The configurator assigns a hole to a band by depth divided by D, at or under one-third, two-thirds and one and a half, and treats anything deeper as the last band; a through hole takes the band of the material it passes through. When the depth is not yet known it uses the two-thirds to one-and-a-half band.

Thread relief undercut for 5/8-18 UNF

A 5/8-18 UNF external thread that runs up to a shoulder ends in a relief groove 0.1668 in wide, three times the pitch, with its root at 0.5499 in, 0.0150 in below the minor diameter. Without it a full-depth single-point thread has nowhere for the tool to stop. Choose an undercut termination on the thread and the configurator derives that groove and prices it as the extra grooving pass it is; choose a runout and the thread simply fades with no groove.

Catalog parts with a 5/8-18 UNF thread

5/8-18 UNF external thread on a shaft end or stud: 3-1/4″ Bore Heavy-Duty Cylinder Tie Rod, 4″ Bore Heavy-Duty Cylinder Tie Rod, 8″ Bore Light-Duty / Pneumatic Cylinder Tie Rod.

Related: All thread sizes · Cylinder parts · Pump shafts · Gearbox shafts.

Frequently Asked Questions

These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard; check them against your part and its mating components before ordering. See all guides on the guides page.