Custom Screws, Bolts and Studs

What does a quote for a special screw or bolt need?

A quote needs three things dimensioned: the thread, the length and the head. The thread is the full callout: size, pitch, series and class. The length is the overall length and the thread length, plus the shoulder diameter and length on a shoulder screw. The head is its style, diameter and height; a knurled thumb screw head is dimensioned like the grip of a custom knob. Add the material, the finish and the quantity. Those numbers on a hand sketch are enough to start a quote; a STEP model helps where the head has shaped geometry.

With no drawing, send a new sample fastener and a note on what it threads into. It can be measured from the sample and drawn, with the drawing quoted as its own line, and the screw is then quoted from that drawing. The sample ships back with the parts.

A shoulder diameter that has to be ground is quoted with precision grinding, and a head profile a cutter cannot reach with wire EDM, on the same order. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Lead time starts at order approval. Put your need date in the part description and the quote answers it.

Special fasteners and other turned parts, page by page

Screws, bolts and studs are quoted down to M1, and no maximum size is published: for a large one, send the print. Three kinds of special have their own pages, and so do four other turned parts. Each page says what its print needs.

Which thread class goes on the callout?

Inch threads are Unified: UNC is the coarse series, UNF the fine and UNEF the extra fine. The class number is the fit. Class 2 is the one most prints call for; class 3 holds the thread closer for precision work; class 1 is the loose fit, for threads that have to go together easily or in dirt. An A after the number means an external thread and a B an internal one, so a bolt is 2A or 3A and its tapped hole 2B or 3B (source 3). A metric callout gives M, the diameter and the pitch in millimetres, then the tolerance class, lowercase (g, h) on an external thread and uppercase (G, H) on an internal one, as in M8 x 1.25-6g. Each diameter has a coarse pitch as its default; the fine pitches are for places a coarse thread would be too deep, such as a thin wall (source 4).

Right-hand is the default, so a left-hand thread carries LH in the callout, after the class: 1/4-20 UNC-3A-LH (sources 5 and 13). Left-hand threads go where the rotation of a shaft would loosen a right-hand nut (source 5). A diameter and pitch outside the standard series is a UNS special, rare on bolts and more common on nuts, tapped holes and threaded ODs (source 3). If the print does not give its major diameter, pitch and class, or its pitch diameter limits, the quote asks for them. A thread with no class on the drawing is quoted as 2A/2B or 6g/6H, and the quote says so. A part can also be threaded inch at one end and metric at the other, each end its own callout in its own system, as on a stud. Thread plug and ring gauges for a special thread are covered on custom thread gauges, and a stud that threads into a welded fitting pairs with weld-in bungs.

What are the 2A and 3A limits on common inch sizes?

The 1957 edition of National Bureau of Standards Handbook H28 (the Bureau is now NIST), Part I, tabulates the limits of size for the standard series of Unified and American screw threads (source 13). The table copies six sizes from its Table III.10: four coarse (UNC) and two fine (UNF), in the two external classes, 2A and 3A. H28 prints the two No. 10 sizes as NC and NF: in 1957, sizes under 1/4 in had not been unified and were known as American Standard threads, with limits on the Unified tolerances and allowances (source 13). Prints today call them UNC and UNF (source 3). A class is a pair of limits, not a nominal size with a plus-or-minus: the diameter has to land between its maximum and its minimum. H28 calls classes 2A and 2B “the most commonly used thread standards for general applications,” and describes 3A and 3B as for applications “where closeness of fit and accuracy of lead and angle of thread are important” (source 13). Fine pitch is a series choice: at the same diameter, 1/4-28 UNF carries more threads per inch than 1/4-20 UNC.

Class 2A has an allowance and class 3A has none. H28 says the 2A maximum diameters apply to an unplated part or to a part before plating, and that the basic diameter, the 2A maximum plus the allowance, applies to a part after plating (source 13). Its own designation examples for a coated thread give the pitch diameter limits before coating and after coating (source 13). If the thread is plated, put on the print which of the two your limits mean.

What are the 2B and 3B limits for a tapped hole?

The same table gives the internal classes. A tapped hole has a pitch diameter that starts at a minimum and runs up to a maximum, and the 2B maximum sits higher than the 3B maximum at every size in the table, so 2B is the looser hole. The minor diameter has its own range. H28 marks its 1B and 2B minor diameter limits as “in process of ratification as Unified Standard” (source 13), so check a minor diameter against the current standard before relying on it.

H28 is a 1957 reference. The current standard is ASME B1.1-2024, “Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms),” which specifies thread form, series, class, allowance, tolerance and designation for Unified screw threads (source 14). We quote to the class on your print.

Which material goes on the print?

For a stainless screw, 303 machines freely because sulfur is added, and that can cost it corrosion resistance around chlorides such as salt water (source 6); a screw that sees salt is commonly specified in 316, the step up from 304 where chlorides cause pitting (source 7). 17-4 PH is the stainless for a screw that needs strength, and it resists corrosion about as well as 304 (source 8). Titanium (Ti-6Al-4V) suits a fastener that has to be light and strong, including around seawater (source 9), and brass suits hardware that must not strike a spark (source 10). Bronze is quoted as well, and so is hardened steel, at the hardness the print calls for. Zinc plating, black oxide and heat treat are quoted with the part. For a plated steel screw, the guide is ASTM B850: it covers “procedures for reducing the susceptibility in some steels to hydrogen embrittlement or degradation that may arise in the finishing processes,” for use “after plating operations,” and its note says the treatment “does not guarantee complete freedom from the adverse effects of hydrogen degradation” (source 11). Baking is quoted when the print calls for it.

Other alloys and plastics quoted on request. Name the grade on the print. A print that says only “steel” gets a quote that names the steel assumed. No C of C or material certifications ship with parts by default. If your PO needs them, say so before the quote. Inspection reports on critical dimensions aren’t included by default. If you need one, ask when you place the order.

Sources

All accessed 27 September 2026, except source 11, accessed 29 September 2026, and sources 13 and 14, accessed 1 October 2026. Where a figure here differs from your print, the print governs.

1. ASME B18.3-2026, "Socket Cap, Shoulder, and Set Screws, and Hex and Spline Keys (Inch Series)": asme.org (socket cap, shoulder and set screws; tabled sizes are not necessarily stock sizes).

2. ASME B18.31.2-2014 (R2019), "Continuous Thread Stud, Double-End Stud, and Flange Bolting Stud (Stud Bolt) (Inch Series)": asme.org (double-end studs; tap-end thread lengths).

3. Wikipedia, "Unified Thread Standard": en.wikipedia.org/wiki/Unified_Thread_Standard (series, classes 1-3, A and B, UNS specials).

4. Wikipedia, "ISO metric screw thread": en.wikipedia.org/wiki/ISO_metric_screw_thread (designation, tolerance class letters, coarse and fine pitch).

5. Wikipedia, "Screw thread": en.wikipedia.org/wiki/Screw_thread (right-hand default, left-hand uses).

6. British Stainless Steel Association, Stainless Steel Advisory Service, "Stainless Steel Fasteners to BS EN ISO 3506, Grades A1, A2 (A3) & A4 (A5)", Information Sheet 2.31, issue 05, 27 September 2007: bssa.org.uk/wp-content/uploads/2022/08/Microsoft-Word-SSAS2.31-Fastener-Grades-A1A2_A3_-A4_A5_-to-BS-EN-ISO-3506-1.pdf (303 with sulfur for free machining, and chlorides).

7. Wikipedia, "SAE 304 stainless steel": en.wikipedia.org/wiki/SAE_304_stainless_steel (316 in place of 304 for chlorides).

8. Wikipedia, "17-4 stainless steel": en.wikipedia.org/wiki/17-4_stainless_steel (high strength, corrosion resistance comparable to 304).

9. Wikipedia, "Ti-6Al-4V": en.wikipedia.org/wiki/Ti-6Al-4V (specific strength, seawater).

10. Wikipedia, "Brass": en.wikipedia.org/wiki/Brass (non-sparking fittings and tools).

11. ASTM B850-98(2022), "Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement": store.astm.org/b0850-98r22.html (scope: some steels and hydrogen embrittlement from finishing processes; use after plating operations; no guarantee of complete freedom from hydrogen degradation).

12. Wikipedia, "Galling": en.wikipedia.org/wiki/Galling (seized threads, stainless, prevention).

13. National Bureau of Standards, "Screw-Thread Standards for Federal Services 1957," Handbook H28 (1957), Part I, issued 10 September 1957 (a U.S. government publication): nvlpubs.nist.gov/nistpubs/Legacy/hb/nbshandbook28supp1957pt1.pdf (Table III.10 limits of size; classes 2A, 2B, 3A and 3B; the 2A allowance and plating; the designation examples and the LH suffix; the footnote on 1B and 2B minor diameters).

14. ASME B1.1-2024, "Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms)": asme.org (scope: thread form, series, class, allowance, tolerance and designation; named here, not reproduced).