Custom Spacers and Standoffs

Catalog standoff or custom spacer: which do you need?

If a distributor lists your thread, length and hex size, buy it there. McMaster-Carr, for one, lists female hex standoffs in inch threads from 0-80 to 5/16-18 and in metric hex sizes from 4.5 mm to 22 mm (source 1), and the quote will point it out when a stock part looks like it would do the job.

When no listing matches the length, OD, thread and material together, the spacer gets turned to the print one at a time or in a batch, with no minimum order. Long, slender standoffs in quantity usually go to Swiss screw machining, where the bar feeds through a guide bushing and the tool cuts close to it (source 2).

Is a spacer turned or milled?

A round or hex spacer is a turned part. The bar rotates while the tool cuts the OD, facing cuts the end perpendicular to the axis of rotation, the hole is drilled and then bored to its final diameter or threaded, and the part is parted off the bar (source 3). Bar stock comes round, rectangular or hex (source 4), so a hex spacer can start from hex bar and keep its flats. A shoulder, a flat, a cross hole or a retaining-ring groove on a round spacer is a feature added to the turned part.

A milled spacer is one whose shape isn’t turned: a square or rectangular block with its faces machined to a thickness tolerance, or a spacer with a pocket or a pattern of holes and slots across its face. Milling cuts flat faces, flat-bottomed pockets and slots with a rotating cutter (source 5), so those are milled from bar or plate. A flat spacer cut from one sheet, with its thickness left at whatever the sheet was rolled to, is a laser part, covered on laser-cut plates, spacers and shims.

Two other parts are easy to confuse with a spacer. A sleeve that presses into a bore or carries a turning shaft is a bushing, one of the round parts on the parts index, and its fits are on custom bushings. A standoff that has to hold in thin sheet metal is usually a self-clinching part, which installs with a squeeze into a properly sized round hole and becomes permanently mounted in the sheet (source 6); hardware insertion covers putting those into a cut part.

How should the length and the bore be toleranced?

Length is usually the dimension that matters most. On a slip spacer it sets the gap in the joint; on a board standoff it sets the space between boards and the clearance under tall components. When several spacers stack to make one gap, their tolerances add: the worst-case method puts every part at its limit (source 7), so three spacers at ±0.005 in each can stack to ±0.015 in, while one spacer made to the full length holds the gap to its own ±0.005 in.

If the ends have to sit square to the bore, put a perpendicularity callout to the bore on the print, and if the OD locates something, a runout callout with the bore as the datum. Runout is read as the part rotates about a datum axis, and total runout covers the entire surface (source 8). Position, perpendicularity and runout callouts are quoted from the print. Concentricity is not the callout to use: ASME Y14.5 eliminated it in its 2018 revision (source 9). For the bore itself, say whether it is screw clearance or a slip fit over a known shaft or shoulder, and give the fit rather than a bare diameter.

How do I call out the thread on a standoff?

Give the full callout for each threaded end. An inch thread takes size, pitch, series and class, as in #6-32 UNC-2B, where class 2 is the most common fit and class 3 the closer one (source 10). A metric thread takes M, the diameter, the pitch and the tolerance class, as in M5x0.8-6H, with uppercase letters for internal threads and lowercase for external (source 11). The coarse metric pitch may be left off a callout (source 11), but writing it removes the question. McMaster-Carr lists its hex standoffs with class 2B or 6H internal threads (source 1), so a 3B callout is one reason to have a standoff made. A thread with no class on the drawing is quoted as 2A/2B or 6g/6H, and the quote says so.

Say whether each tapped hole goes through or stops blind. A plug tap has 3 to 5 tapered threads at its tip and a bottoming tap 1 to 1.5 (source 12), so the bottom of a blind hole carries incomplete threads, fewer when it is finished with a bottoming tap. Give the full-thread depth the mating screw needs, not only the hole depth. On a male-female standoff, dimension the male end as you would a stud: its thread, its length and any relief at the shoulder. Custom fasteners covers thread series and classes in more depth. A different thread at each end, inch on one and metric on the other, is quoted like any other callout.

Which material and finish suit a spacer?

6061 aluminum is where most spacers start. A thin-wall spacer under heavy clamp load steps up to 7075: typical yield is 39 ksi for 6061-T6 (source 13) against 73 ksi for 7075-T651 (source 14), and the 6061 vs 7075 comparison covers when the step is worth it. Brass is used where a spacer has to conduct and be soldered for grounding, and stainless where corrosion resistance matters (source 15); the 303 and 304 and 316 grade pages cover which stainless to name. Delrin (acetal) is a dielectric (source 16), so it insulates a board from a chassis. Other alloys and plastics quoted on request. Name the grade on the print.

A finish called out on the print is quoted with the part. Typical anodize grows into and out of the surface by equal amounts, and a tapped hole can bind afterward unless it is chased with a tap (source 17), so the print should say whether a thread class applies after finish. The anodic layer does not conduct (source 17), so a face that has to ground a board is masked to leave bare metal, or the standoff is specified in brass. Stainless ships bare or passivated as the print calls for, and Delrin ships as machined.

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.

Sources

All accessed 27 September 2026. Where a figure here differs from your print, the print governs.

1. McMaster-Carr, "Female Threaded Hex Standoffs": mcmaster.com/products/female-threaded-hex-standoffs/ (inch threads 0-80 to 5/16-18; metric hex sizes 4.5 to 22 mm; classes 2B and 6H).

2. Peter Zelinski, "Understanding Swiss-Type Machining", Modern Machine Shop, 26 June 2013, updated 14 July 2022: mmsonline.com/articles/understanding-swiss-type-machining (guide bushing support; long, slender turned parts).

3. Wikipedia, "Turning": en.wikipedia.org/wiki/Turning (facing, boring, threading, parting off).

4. Wikipedia, "Bar stock": en.wikipedia.org/wiki/Bar_stock (round, rectangular and hex bar).

5. Wikipedia, "Milling (machining)": en.wikipedia.org/wiki/Milling_(machining) (flat faces, pockets and slots).

6. PennEngineering, "Standoffs for Sheet Metal", PEM catalog: catalog.pemnet.com/category/standoffs-for-sheet-metal (self-clinching standoffs in thin sheet).

7. Wikipedia, "Tolerance analysis": en.wikipedia.org/wiki/Tolerance_analysis (worst-case stack-ups).

8. Seth Elder, "Circular Runout vs. Total Runout", GD&T Basics, 14 September 2020: gdandtbasics.com/circular-runout-vs-total-runout/ (runout read as the part rotates about a datum axis; total runout over the entire surface).

9. Wikipedia, "Geometric dimensioning and tolerancing": en.wikipedia.org/wiki/Geometric_dimensioning_and_tolerancing (concentricity eliminated in the 2018 revision).

10. Wikipedia, "Unified Thread Standard": en.wikipedia.org/wiki/Unified_Thread_Standard (designation; classes 2 and 3).

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

12. Wikipedia, "Tap and die": en.wikipedia.org/wiki/Tap_and_die (plug and bottoming taps; blind holes).

13. Wikipedia, "6061 aluminium alloy": en.wikipedia.org/wiki/6061_aluminium_alloy (6061-T6 typical yield, 39 ksi).

14. Wikipedia, "7075 aluminium alloy": en.wikipedia.org/wiki/7075_aluminium_alloy (7075-T651 yield, 73,000 psi).

15. RS Components, "A Guide to Spacers and Standoffs", 8 September 2026: uk.rs-online.com/web/content/discovery/ideas-and-advice/spacers-standoffs-guide (spacer and standoff types; brass and stainless uses).

16. Wikipedia, "Polyoxymethylene": en.wikipedia.org/wiki/Polyoxymethylene (POM is a dielectric; Delrin).

17. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (dyeing; coating growth and threads; insulation).