Aerospace Pins: Dowel, Cotter, Clevis, Spring and Quick-Release
Aerospace pins locate parts, carry shear or retain other hardware, and are bought by standard part number. Dowels are NASM16555 and NASM16556, cotter pins NASM24665, clevis pins NASM20392, quick-release pins NASM17984 and NASM17985, and spring pins NASM16562 and NASM39086, all former MS standards that kept their part numbers.
Updated
Pin types and where they are used
Each pin type has one job, and the FAA's guidance on where it may and may not be used is as important as the part number.
| Pin | Standard | Job | Use limits from FAA AC 43.13-1B and the General handbook |
|---|---|---|---|
| Dowel, straight headless | NASM16555 (0.0002 in over nominal), NASM16556 (0.001 in over nominal) | Locate mating parts precisely | Fit set by the oversize and the reamed hole |
| Cotter (split) | NASM24665; AN380 and AN381 legacy | Retain castellated nuts, clevis pins, bolts | Do not reuse. Use CRES cotters where nonmagnetic or corrosion resistance is needed |
| Clevis (flathead, drilled shank) | NASM20392 | Hinge tie-rod terminals and secondary controls | Not for continuous operation; install head up or forward so it stays if the cotter fails |
| Quick-release, positive locking | NASM17984 (button handle), NASM17985 (T handle) | Equipment that must come off fast | No spindle binding, or the balls can stay open and the pin can vibrate out |
| Spring, slotted or coiled | NASM16562 (slotted), NASM39086 (coiled heavy duty); MS9047 and MS9048 also listed | Double shear retention without other locking | Not where loss would endanger safe flight; not mixed with other structural fasteners in one joint; not a cotter substitute |
| Taper, plain or threaded | AN385, AN386 with AN975 washer | Shear joints where no play is allowed | Plain pins are drilled and safety wired |
AC 43.13-1B adds that spring pin performance depends entirely on the fit and its durability under vibration, especially in soft aluminum and magnesium, and that a spring pin may be reused only if inspection shows no deformation of the pin or hole.
The specifications that govern pins
The MS pin standards below were superseded by AIA NASM standards, with cancellation notices dated mostly from the late 1990s to 2000. The MS16555C cancellation notice states the rule plainly: the NASM part numbers are identical to the MS numbers, so MS16555-1 stays MS16555-1. Status below is from EverySpec as of October 2026.
| Former document | Title | Status |
|---|---|---|
| MS16555C | Pin, straight, headless (dowel), 0.0002 over nominal | Cancelled 16 March 2000, superseded by NASM16555 |
| MS16556B | Pin, straight, headless (dowel), 0.001 over nominal | Superseded by NASM16556 |
| MS24665F | Pin, cotter (split) | Superseded by NASM24665 |
| MS20392F | Pin, straight, headed, drilled shank (clevis), superseding AN394 | Superseded by NASM20392 |
| MS17984F | Pin, quick release, self-retaining, positive locking, single acting, button handle | Superseded by NASM17984 |
| MS17985G | Same family, T handle | Superseded by NASM17985 |
| MS16562C | Pin, spring, tubular, slotted | Superseded by NASM16562 |
| MS39086B | Pin, spring, tubular, coiled, heavy duty | Superseded by NASM39086 |
AC 43.13-1B Table 7-15 also lists MS9047 (spring pin, steel, phosphate finish), MS9048 (spring pin, steel, cadmium plate), MS9245 (cotter pin, CRES, AMS 7211), MS9390 (straight headless dowel pin, CRES, AMS 5735, standard and oversize) and MS35671 through MS35679 (grooved, headless, tapered groove). We could not confirm the current status of the MS9000 series documents, so check each one on ASSIST before you cite it.
Have a part like this?Send it in under a minute. A person replies within 1 business day.How a pin part number is built
Pin standards use the basic number plus a dash number from a table in the standard, sometimes with a material letter. The tables are not intuitive, so read the dash number from the current sheet rather than guessing from a neighbor.
MS16562C gives its own example: MS16562-1 is a slotted tubular spring pin of 0.062 inch nominal size in carbon steel, cadmium plated. The size, length and finish all come from the dash number. MS39086B uses the same scheme: MS number plus dash number. The dowel notices use MS16555-1 as their cross-reference example and confirm the number did not change when NASM16555 took over.
Because the decoding lives in tables we cannot reproduce here, put the full dash number, the standard and its revision on the PO and let the supplier's cert repeat it.
Materials, hardness and tolerance classes
Pin materials are hard steels, and hardness is the property you can actually check at receiving.
| Standard | Material | Finish | Hardness |
|---|---|---|---|
| MS16562 slotted spring pin | Carbon steel grades 1070 to 1095, or CRES 410 and 420 | Carbon steel cadmium plated or phosphate coated; CRES cleaned, descaled and passivated | Carbon steel 46 to 53 HRC; CRES 43 to 52 HRC |
| MS39086 coiled heavy duty spring pin | Carbon steel 1070 to 1095, or CRES 302, 410 or 420 | Carbon steel cadmium plated per QQ-P-416 Type II Class 3; CRES passivated | Set in the standard by material; 302 is work hardened |
| Cotter pins, AN380 | Low carbon steel | Cadmium plated | Not a hardness-controlled part |
| Cotter pins, AN381 and MS24665 CRES | Corrosion resistant steel | None | Nonmagnetic, corrosion resistant |
The values above are from MS16562C and MS39086B as published on EverySpec; NASM revisions may have changed finishes, especially cadmium. Many programs now restrict cadmium, so confirm the finish your drawing allows and state it on the PO.
Dowel tolerance classes
The two common dowel standards differ only in oversize. NASM16555 pins are 0.0002 inch over nominal and NASM16556 pins are 0.001 inch over nominal, per the MS titles. The hole tolerance on the drawing has to match the pin, so a dowel swap is an engineering change, not a buyer's choice.
Trade-offs
- Carbon steel spring pins are strong but need plating or phosphate for corrosion; CRES trades a little hardness for corrosion resistance.
- Galvanic contact: steel pins in aluminum structure are a dissimilar pair. See surface finishing and stainless and specialty steels.
- Finish versus environment: cadmium plate, phosphate coating and passivation are the finishes these standards list for different materials; take the finish from the drawing rather than from what is on the shelf.
Installation rules that affect what you buy
- Cotter pins are single use. AC 43.13-1B says they should not be reused, so buy enough for every reassembly, not just the first build.
- Clevis pins go head up. Order the clevis pin and its cotter as a pair, with the cotter sized to the drilled shank.
- Spring pins need a correct hole. AC 43.13-1B says if hole misalignment closes the pin gap or needs excess inserting force, the spring pin will not be used. A drawing change may be cheaper than a pin substitution.
- Spring pins can be nested. AC 43.13-1B notes spring pins can be used inside one another to increase shear strength, which only applies when the drawing calls both.
- Quick-release pins must move freely. Check the spindle and locking balls at receiving and before installation.
- Taper pins are matched to a reamed taper, and threaded taper pins are used with an AN975 washer and a shear nut or self-locking nut.
For the safety wire and cotter pin practice that secures these pins, see panel fasteners and safety wire.
What to put on the PO
- Full part number with dash number, the NASM standard and revision, and any finish letter.
- Manufacturer and lot traceability on the cert and package.
- Certificate of conformance to the standard, with hardness results where the standard sets hardness. See certificates of conformance.
- Finish restrictions, for example no cadmium, when the program requires it.
- DFARS specialty metals: CRES pins and many alloy steel pins are covered metals under 252.225-7009 when the contract carries the clause. See DFARS specialty metals.
- Packaging: one part number per labeled bag; dowels protected from handling damage on the ground diameter.
- No substitution between dowel oversizes, spring pin styles or cotter materials without approval.
Counterfeit and quality risks, and how to receive pins
Pins are commodity-looking parts that are easy to replace with commercial hardware of unknown hardness. AS6174 covers counterfeit materiel of any type, including mechanical hardware.
- Hardness check on a sample of spring pins against the range in the standard.
- Measure dowel diameter with a calibrated micrometer; the difference between the two dowel standards is less than a thousandth, so a caliper is not enough.
- Check material and magnetism on cotter pins ordered as CRES; a magnet test catches plain steel substitutes, though some CRES grades are magnetic, so treat it as a screen, not an acceptance test.
- Function test quick-release pins: the button or T handle should release the balls fully and the balls should lock fully.
- Reject unlabeled or mixed lots.
See receiving inspection and counterfeit parts avoidance.
How we source pins
We do not keep pin inventory, publish prices or act as an authorized distributor for any maker. We source pins through authorized and franchised distributors or the manufacturer, flow down the standard, finish and cert requirements, and check hardness or dimensions when your PO asks for it. Pins are rarely bought alone: we can put them on one PO with the machined parts or ground support equipment they install in, and build kits with the bolts, castellated nuts and washers they retain. Discontinued quick-release pins and odd dowel sizes go through our obsolete parts service. Related: rivets, threaded inserts, and the fasteners hub.
Get pins sourced or kitted
Send part numbers, quantities and any finish restrictions through a hardware quote request. A person replies within one business day with one quote and one lead time, even when the pins ride with machined or fabricated parts, and every order ships with a certificate of conformance.
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Part, quantity and date are enough to start. Drawings and the mutual NDA follow by reply, and controlled programs never send technical data through this site.
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Questions
Is MS24665 still a valid cotter pin number?
Yes, as a part number. EverySpec lists MS24665F as superseded by NASM24665, and the MS to NASM conversions kept the original part numbers. Cite NASM24665 and its revision as the governing document on new drawings and POs while ordering the familiar MS24665 dash number. Remember that AC 43.13-1B says cotter pins should not be reused.
What is the difference between MS16555 and MS16556 dowel pins?
Oversize. MS16555, now NASM16555, is a straight headless dowel 0.0002 inch over nominal size, and MS16556, now NASM16556, is 0.001 inch over nominal. The reamed hole on the drawing is toleranced for one of them, so they are not interchangeable without an engineering change. Check diameter at receiving with a micrometer.
Can spring pins be used in flight-critical joints?
AC 43.13-1B says spring pins should not be used in an aircraft component or system where loss or failure of the pin might endanger safe flight, should not be mixed with other structural fasteners in one joint, and should not replace a cotter pin. For primary structure they should be used only where there is no rotation or relative movement. Follow the drawing or repair manual.
What should I check on quick-release pins at receiving?
Function first. The pin should release fully when the button or T handle is operated and the locking balls should extend fully when released, with no binding. AC 43.13-1B warns that spindle binding can leave the balls open so the pin falls out under vibration. Then check diameter, grip length and the cert against the NASM17984 or NASM17985 dash number.
Which way should a clevis pin be installed?
Head up, or head forward. The FAA General handbook and AC 43.13-1B both say the flathead clevis pin is customarily installed with the head up so that if the cotter pin fails or works out, the pin stays in place. Order the matching cotter pin size for the drilled shank with every clevis pin.
Are steel pins covered by DFARS specialty metals?
Often. CRES pins and many alloy steels exceed the alloy thresholds that define specialty metal, so on a defense contract carrying DFARS 252.225-7009 they must come from a compliant melt or fit an exception in the clause. Low carbon steel cotter pins may fall below the thresholds; check the chemistry rather than assuming.
Related
Sources
- FAA AC 43.13-1B, Chapter 7 Sections 6 and 7, pins and safetying, Table 7-15
- FAA-H-8083-30A Aviation Maintenance Technician Handbook, General, pins and safetying
- EverySpec, MS16555C Notice 2, cancellation and NASM16555 cross reference
- EverySpec, MS16556B superseded by NASM16556
- EverySpec, MS24665F superseded by NASM24665
- EverySpec, MS20392F superseded by NASM20392
- EverySpec, MS17984F superseded by NASM17984
- EverySpec, MS17985G superseded by NASM17985
- EverySpec, MS16562C slotted spring pin, superseded by NASM16562
- EverySpec, MS39086B coiled heavy duty spring pin, superseded by NASM39086
- SAE AS6174, Counterfeit Materiel
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