Pins — a diameter tolerance instead of a thread tolerance
Pins have no property class and no thread. What they have is an m6 diameter band eight microns wide that a zinc deposit fills on its own — plus ISO 8734 hardness, double shear for spring pins, and a strong case for phosphate.
At a glance
| Thread / dimensional standard | ISO 8734 · m6 diameter fit |
|---|---|
| Tolerance class | m6 (dowel) · h8 (taper) |
| Barrel window | ø1.5 – ø13, up to ≈ 60 mm |
| Rack / basket | ≥ ø16 or long pins that bow |
| Strength specification | no property class — double shear + m6 |
| ISO 4042 relief bake | Usually required if electroplated |
| Reviewed | 2026-08-04 |
Pins locate and shear. They have no thread, no property class and no proof load. What they have instead is a ground diameter held to m6 — a band about eight microns wide on a six-millimetre pin — and any coating goes straight into it. That single constraint decides almost everything else on this page.
Why this shape is processed the way it is
A dowel pin is a ground cylinder whose entire function is an interference or transition fit, so straightness, roundness and surface finish are functional characteristics rather than cosmetic ones. A spring pin is the opposite idea: a rolled sleeve deliberately made oversize that closes as it is driven, so its finish must not stop it compressing. The same word covers two parts with opposite tolerance philosophies, and a drawing that does not name the standard is ambiguous.
Threads, tolerances and dimensional standards
The governing standards are dimensional, not thread standards. ISO 2338 covers unhardened parallel pins, ISO 8734 hardened dowel pins, ISO 8735 parallel pins with an internal thread for extraction, and ISO 2339 taper pins. Spring pins split by construction: ISO 8752 and ISO 13337 for slotted types, ISO 8748 for coiled. ISO 1234 covers split pins, and JIS B 1354 is the Japanese parallel pin series. Dowel pins are normally m6; taper pins work to h8 on the small diameter.
Size band and handling route
Diameters from 1.5 to 13 mm and lengths up to roughly 60 mm run in a barrel. Longer pins bow against each other under load and are racked, or more often left uncoated. Slenderness rather than diameter is the real limit — a long thin pin picks up a permanent bend from its own weight in a hot barrel long before it picks up a coating problem.
Strength specification
There is no property class for pins and no tensile class to certify, because the load case is double shear rather than tension. Hardened dowel pins to ISO 8734 are supplied hard, which buys wear resistance and spends almost all of the available toughness — a hardened dowel pin does not bend before it breaks, and it is not supposed to. Spring pins are specified instead by a minimum double-shear load in ISO 8752 and ISO 13337, which is a function of the sleeve wall thickness and the material, not of a hardness figure.
No property class exists for this part type. Anything on a drawing that reads like one is a specification error worth fixing before the order runs.
Hardness: ISO 8734 hardened dowel pins: high-50s to mid-60s HRC (≈ 600–800 HV)
Coating thickness and what limits it
This is the pin version of the thread allowance problem, and it is tighter. An m6 fit on a six-millimetre pin is a band eight microns wide, and electroplating adds twice the coating thickness to a diameter — so a deposit that would be considered thin on a bolt consumes the entire fit on its own. In practice hardened dowel pins are supplied plain and oiled, ground after coating, or given a coating that barely builds. Phosphate is the usual answer, because a phosphate layer grows partly into the substrate rather than only on top of it and therefore costs less diameter for the protection it gives.
| Feature | ø6 mm dowel pin, m6 |
|---|---|
| Available band | +12 / +4 µm — an 8 µm band |
| Coating budget | coating adds 2× thickness to a diameter |
- ASTM B633 SC 1 — Fe/Zn 5 (only where the fit allows)
- phosphate + oil where it does not
Hydrogen embrittlement relief
A hardened dowel pin is far above the ISO 4042 threshold of about 390 HV and has almost no ductility left to absorb a crack once one starts. If such a pin is electroplated, relief within four hours is not a preference. The more common and more honest decision is not to electroplate it at all: phosphate and oil, or a bare ground surface, avoids the hydrogen entirely and keeps the fit. Unhardened ISO 2338 pins are a different case and sit comfortably below the threshold.
Hydrogen de-embrittlement bake checkerHow this part fails
- Pin snaps instead of bending
A through-hardened dowel pin has essentially no ductility. Any load it was not designed for — a misaligned assembly, a hammer, a shock — breaks it cleanly rather than deforming it, and the fracture face looks brittle because it is.
- Will not press into the hole after coating
The diameter grew by twice the coating thickness and the m6 band is gone. The pin gauged correctly before finishing and interferes afterwards, which is why pin drawings should state whether the tolerance applies before or after coating.
- Quench crack along the length
High-carbon and bearing-steel pins crack longitudinally during quenching, often invisibly until the pin is pressed in. Section change, quench severity and delay before tempering are the usual contributors.
- Spring pin loses radial force
A slotted or coiled pin that has been over-tempered, or heated in a bake chosen for a different part, no longer springs back against the hole wall. It installs normally, holds nothing, and works loose.
What to check on the lot
- 1Diameter against the m6 band, after coatingThe measurement that matters is on the finished pin. State on the drawing whether m6 applies as-ground or as-delivered, because the two are different parts once anything has been deposited.
- 2Hardness by a method the section can carryRegular Rockwell to ISO 6508-1 works on larger diameters; small pins need superficial HR15N or a Vickers reading on a mounted section per ISO 6507-1. A curved surface also biases a Rockwell reading unless a correction is applied.
- 3Surface roughness and straightnessA dowel pin is a fit, so finish and straightness are acceptance characteristics. A rough or bowed pin galls the hole on installation and destroys the location it was supposed to provide.
- 4Double shear on spring pinsISO 8752 and ISO 13337 specify a minimum double-shear load. That test, not a hardness number, is what proves a spring pin — and it is the one most often left off the drawing.
Test methods
Standards cited on this page
Questions engineers ask
Can you zinc electroplate hardened dowel pins?+
Technically yes, and usually it is the wrong choice. The deposit adds twice its thickness to the diameter and an m6 band on a small pin cannot absorb that, and the hardness sits well above the ISO 4042 relief threshold so the lot needs de-embrittlement within four hours. Phosphate and oil keeps the fit and skips the hydrogen. Tell us the fit class and we will say which route we would run.
What hardness should a dowel pin be?+
ISO 8734 supplies hardened pins in a high band chosen for wear rather than toughness, with the exact range depending on the type and the material. If the pin has to survive shock or misalignment, that hardness is a liability and an unhardened ISO 2338 pin, or a case-hardened pin with a tough core, is the better specification.
Why do our pins crack when we press them in?+
Either the pin is right and the hole is wrong, or the pin has a crack it arrived with. Check the hole tolerance and alignment first, because a hardened pin entering a misaligned or undersized hole is being asked to bend. If the holes are correct, section the failed pins: a quench crack from heat treatment and a hydrogen crack from plating both show up on a fracture face and point at completely different fixes.
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Guidance based on published standards and general fastener metallurgy. Every figure is a typical range that depends on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. Coating designations describe what a drawing can call for, not a result guaranteed on your parts without testing.