Guide
Custom Shoulder Screws: Which Shoulder Tolerance and Thread Do You Specify?
Shoulder screws are fasteners with a cylindrical shoulder between the head and a smaller thread, used as pivots, guides and locating pins that also clamp. The shoulder diameter is what the part is bought for, so a custom drawing has to state the shoulder tolerance, the thread and the thread relief separately instead of citing a standard number alone.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers drawing a custom shoulder screw or deciding whether a catalog part will do. It compares what the metric ISO 7379 and the inch ASME B18.3 families fix, shows why catalog tolerance data can disagree, and ends with a callout and RFQ checklist. Both standards are paid documents, so values below come from the linked sources or are labeled typical; read the clauses in your own licensed copy before copying anything onto a drawing.
On this page
What do ISO 7379 and ASME B18.3 actually fix on a shoulder screw?
ISO 7379 is the metric standard for hexagon socket head shoulder screws, and ASME B18.3 is the inch standard that covers shoulder screws alongside socket cap and set screws. Both tie each shoulder diameter to one thread size, but they use different shoulder sizes, different tolerance bands and different thread families.
Per ISO 7379:1983, the metric shoulder diameters run from 6.5 to 25 mm and are paired with M5 to M20 threads (fasten.it dimension listing, edition dated October 1983). Table 1 lists the pairings. The standard is old, and distributors still sell to it, so the part number on a drawing is rarely the whole specification.
ASME B18.3 is the inch counterpart. Its shoulders are fractions of an inch, and the thread is normally one standard size smaller than the shoulder, as summarized from the B18.3-2003 tables by AFT Fasteners.
A metric 8 mm shoulder and an inch 5/16 in shoulder are close in typical catalog size, but they are different parts on different tolerance bands. Even inside one family a label can hide a difference: Würth sells screws it calls WN 7379 whose shank is 6 mm for M5, 12 mm for M10 and 24 mm for M20, which it says differ from ISO 7379 (Würth Finland listing). Write the shoulder diameter and tolerance on the drawing, not just the standard number.
| Shoulder d1 (mm) | Thread | f9 limits (µm) | f9 band width (µm) |
|---|---|---|---|
| 6.5 | M5 | −13 to −49 | 36 |
| 8 | M6 | −13 to −49 | 36 |
| 10 | M8 | −13 to −49 | 36 |
| 13 | M10 | −16 to −59 | 43 |
| 16 | M12 | −16 to −59 | 43 |
| 20 | M16 | −20 to −72 | 52 |
| 25 | M20 | −20 to −72 | 52 |
How different are the shoulder tolerances in metric and inch catalogs?
The metric shoulder is sold as ISO class f9, a band that moves and widens with diameter. The inch shoulder is a negative band, 0.002 to 0.004 inch under nominal for a 1/4 inch shoulder per ASME B18.3-2003 as tabulated by AFT.
Distributors describe the metric shoulder as class f9 (the Würth listing says "tolerance class f9"), which is why Table 1 gives f9. At 8 mm, derived per ISO 286 (the f upper deviation, which the RoyMech ISO 286-2 table shows as −13 µm for sizes over 6 up to 10 mm, plus the IT9 width), the f9 limits work out to −13 to −49 µm. A large distributor's metric listing instead shows −0.072 to −0.010 mm for shoulders from 8 to 16 mm and −0.080 to −0.020 mm for some 20 and 24 mm shoulders (McMaster-Carr, ISO 7379 listing), a different pattern from Table 1. Treat a catalog tolerance as that catalog's number, not as the standard's.
For inch screws the same distributor lists the 1/4 inch shoulder at −0.004 to −0.002 inch (McMaster-Carr, inch listing), matching the B18.3-2003 limits of 0.2480 and 0.2460 inch for the 1/4 inch size (AFT Fasteners).
Table 2 puts the two families side by side at the same nominal size. On these numbers the inch shoulder sits further under nominal and has a wider band than a derived f9 band at the same size would, so the clearance in the same reamed hole is likely to differ. Because the metric row is derived and real catalogs vary, check the actual parts before swapping families; redrawing the bore may be needed, and the guide to reamed and bored H7 holes covers the hole side.
| Family | Shoulder limits | Deviation from nominal (µm) | Band width (µm) |
|---|---|---|---|
| Inch, ASME B18.3 | 0.2460 to 0.2480 in | −102 to −51 | 51 |
| Metric f9, same nominal size | 6.301 to 6.337 mm | −49 to −13 | 36 |
How does the thread size follow the shoulder diameter?
Per ISO 7379 the thread is one step smaller than the shoulder (M5 on the 6.5 mm shoulder), and per ASME B18.3 the inch thread is one standard size smaller (10-24 on a 1/4 inch shoulder). On a custom screw the thread is a free choice, so state it and its class explicitly.
Inch pairings in one catalog run 4-40 on a 1/8 inch shoulder, 8-32 on 3/16 inch and 10-24 on 1/4 inch, with a Unified Class 3A thread (McMaster-Carr). The AFT summary of B18.3-2003 adds that the one-size-smaller rule has stated exceptions, such as a 3/8-16 thread on a 1/2 inch shoulder and a 1/2-13 thread on a 5/8 inch shoulder (AFT Fasteners).
The step down in size has a cost. The thread is much smaller than the shoulder, and Würth notes that these screws can only be loaded to a limited extent in the pull-out test under ISO 898-1, with the strength class marked with a leading 0 (Würth Finland). If your joint carries tension through the thread, size the thread for the load rather than inheriting it from a catalog shoulder.
Call the thread the way you would on any turned part: size, pitch, tolerance class and usable length. The thread callout guide shows the wording.
Which shoulder and thread features does a custom drawing need to control?
Four features decide whether a custom shoulder screw works: the shoulder diameter and length, the relief at the step to the thread, the fillet under the head, and the coaxiality of shoulder and thread. A catalog screw inherits all four from the standard; a custom screw gets only what the drawing states.
Per ASME B18.3-2003, as tabulated by AFT Fasteners, the inch standard controls a thread neck with maximum and minimum diameters, a neck width and a fillet, and a shoulder neck beneath the head. For the 1/4 inch size, per ASME B18.3-2003, the thread neck is listed at 0.142 inch maximum and 0.133 inch minimum, with a basic width of 0.083 inch. In the same tables the length tolerance is typically −0.02 inch up to 3/8 inch and −0.03 inch above it.
The relief is what lets the shoulder seat flat against a counterbore floor. Without it, the last thread runout and the corner at the base of the shoulder both interfere. The thread relief groove guide covers sizing the groove.
Coaxiality matters because the shoulder locates the part and the thread only clamps it. Make the shoulder the datum and call the thread position or runout against it, with a value taken from the function. Turning the shoulder and the thread in one chucking is the usual way to keep them coaxial, and it is a question worth asking in the RFQ.
When is a catalog shoulder screw enough, and when should you go custom?
A catalog screw is enough when the shoulder diameter and length are standard sizes, the standard band suits the bore, and the material and finish are stock options. Go custom when any of those must change, because then the drawing sets the shoulder, the length and the relief.
Table 3 sorts the common triggers. At a steady annual volume in the tens of thousands, the unit cost depends more on the design than on one-off drawing effort, so it is worth quoting the nearest catalog part and a custom drawing side by side.
Saola turns custom pins and shoulder-style parts from bar on CNC turning lathes at its lead CNC plant. Heat treatment or a finish, when the drawing calls for one, is arranged through partner plants that Saola manages, and the quote names the production site.
| Situation | Catalog part | Custom drawing |
|---|---|---|
| Shoulder is a standard diameter and the standard band suits the bore | Usually enough | Not needed |
| Shoulder falls between standard sizes, or metric and inch tooling are mixed | Does not fit | Set the diameter and tolerance on the drawing |
| Shoulder length is not one of the catalog steps | Does not fit | Give the length and its tolerance |
| The bore needs a tighter or looser shoulder band than the standard gives | Standard band only | State class or limits |
| Material, heat treatment or finish is not stocked | Limited choice | Name them on the drawing |
| Extra features such as a cross hole, flat, lock patch or special head | Not available | Draw the feature and its datum |
How should the drawing and the RFQ describe a custom shoulder screw?
Give the shoulder diameter with a class or limits, the shoulder length with a tolerance, the thread with its class, the relief and fillet, and a coaxiality note. Then ask the supplier to confirm which standard edition any catalog dimensions come from.
- Shoulder: nominal diameter plus either a class such as f9 per ISO 286 or explicit limits. Limits written on the drawing should override any catalog default.
- Length: shoulder length and thread length, each with a tolerance, and the head height if the head seats in a counterbore.
- Thread: size, pitch, tolerance class and usable length.
- Features: thread relief, shoulder-to-head fillet or undercut, and the head and drive style.
- Datum: shoulder as datum A, with thread coaxiality called against it.
- Material and finish: alloy, strength class if it matters, heat treatment and any coating.
- Inspection: shoulder diameter measured at more than one section, and the thread checked with go/no-go gauges.
A short note block such as SHOULDER Ø8 f9 PER ISO 286-2; THREAD M6x1-6g; RELIEF PER VIEW A; THREAD COAXIAL TO A is easier to quote than a catalog part number. Attach the drawing to a request through the quote form and say whether a catalog screw is an acceptable fallback.
Frequently asked
Is the ISO 7379 shoulder always class f9?
Distributors describe it as f9, for example in the Würth listing, but numeric limits in retail listings do not always match the f9 values in Table 1. Confirm against your licensed copy of the standard and put the limits on the drawing.
Can I use an inch shoulder screw in a metric reamed hole?
Check the fit first. A 1/4 inch shoulder is 6.35 mm, which is not a typical metric shoulder size, and Table 2 shows the inch band sits further under nominal than f9. Mixing families usually means a custom shoulder or a redrawn bore.
Do I need a thread relief on a shoulder screw?
The inch standard tabulates a thread neck and fillet for exactly this reason, and the same logic applies to metric screws. Call out a relief or ask the supplier to propose one; the thread relief groove guide linked above covers the sizing.
How is the shoulder length toleranced?
Per ASME B18.3-2003 as tabulated by AFT, the inch length tolerance is −0.02 inch up to 3/8 inch and −0.03 inch above it (AFT Fasteners). On a custom drawing, set the length tolerance from how the shoulder seats in the mating part.
Can Saola quote a custom shoulder screw?
Yes, as a turned part from bar. Send the drawing with the shoulder class or limits, the thread callout and the volume you expect per year, and the quote will state the production site and country of origin.
Sources
- fasten.it, ISO 7379 standard dimensions (listing of ISO 7379:1983), dimension table, accessed October 2026
- Würth Finland, ISO 7379 tolerance f9 steel 12.9 listing, product page, accessed October 2026
- McMaster-Carr, shoulder screws meeting ISO 7379, catalog page, accessed October 2026
- McMaster-Carr, inch shoulder screws, catalog page, accessed October 2026
- AFT Fasteners, socket shoulder screws dimensions (ASME B18.3-2003 tables), technical page, accessed October 2026
- ISO 286-1:2010, Geometrical product specifications (GPS), ISO code system for tolerances on linear sizes, Part 1, ISO, 2010 (source of the IT9 widths used in the derived f9 limits)
- RoyMech, ISO 286-2 shaft limits, reference table, accessed October 2026