Aerospace Sourcing
Parts: fasteners

Aerospace Washers: Flat, Countersunk, Lock, Tab and Key

Aerospace washers are bought by standard part number: NAS1149 flat washers, which replaced the AN960 numbering, countersunk washers such as NAS143C and MS20002C, NASM35338 split lock washers and MS tooth, tab and key washers. FAA guidance bars lock washers from primary and secondary structure, so check the drawing before substituting.

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Washer types and where they are used

A washer in an aerospace joint does one of four jobs: spreads the bearing load and protects the surface, shims the grip, seats a bolt head radius, or locks the joint. FAA AC 43.13-1B divides them into plain, lock and special washers, and its Table 7-14 lists the standard numbers. The families below come from that table and paragraphs 7-85 to 7-89.

FamilyDescriptionTypical use
AN960, now NAS1149Flat washerBearing surface under nuts and heads, grip shimming, positioning castle nuts
AN970Flat, larger bearing areaWooden structure, to prevent crushing
NAS620Flat, reduced outside diameterTight spaces next to flanges and radii
NAS1197Flat, 5052 aluminumAluminum structure where a steel washer is not wanted
NAS549Flat, resin fiberInsulating or nonmetallic bearing surfaces
NAS143, NAS143C, MS20002, MS20002CPlain and countersunk high strength washersUnder internal wrenching bolts and nuts
NAS1169Dimpled 100 degree washerCountersunk screws in thin or dimpled sheet
MS27111, MS27129Countersunk finishing washersCosmetic and bearing surface under countersunk screws
AN935, MS35337 to MS35340Split spring lock washers, light, medium and heavyMachine screws and bolts where a locknut or castle nut cannot be used
AN936, MS35333 to MS35336, MS35790Internal and external tooth lock washers, flat and countersunkElectrical bonding points and light secondary hardware
NAS460; MS9276, MS9581, MS9582; MS9684Tab washers; key washers with 180, 90 and 270 degree locking; cup lock washersLocking nuts and bolts mechanically by bending a tab or key
AN950, AN955Ball socket and ball seat washers, used as a pairBolts installed at an angle to the surface
NAS1099Bevel washer, 9 1/2 degreesFlanges with a tapered bearing face
NAS1061High temperature spring lock washerHot locations

The specifications that govern washers

Washers are defined by their part standards, and the two most common numbers have both changed owners. The statuses below are the ones we confirmed; for any other number, check DLA ASSIST or the issuing body.

SpecWhat it coversStatus as verified
AN960Flat washerNumbering superseded by NAS1149; distributors list AN960 numbers as alternates to specific NAS1149 numbers
NAS1149Flat washer, multiple materials and finishesCurrent number for new orders
MS35338Split lock washerRev. L (1994) superseded by NASM35338 in 2002; part numbers identical
NASM35338Split lock washerCurrent; listed as the 2014 edition by SAI Global
AN935, AN936Spring lock and tooth lock washersListed in AC 43.13-1B Table 7-14; check status before new design
MS9276, MS9581, MS9582Corrosion resistant steel key washers to AMS 5510Listed in AC 43.13-1B Table 7-14; check status before new design

The washer itself is only half the callout. Material and finish come from the standard and the letter codes in the part number, so a steel washer with the wrong code can be the right size and still be the wrong part.

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How the part number is built

NAS1149 numbers pack material, size, thickness and finish into one code, which is why they replaced the shorter AN960 numbers. The most useful thing a buyer can do is read the cross-reference pairs that distributors publish, then confirm against the standard:

Old numberNAS1149 number listed as alternateWhat the listing says
AN960-10NAS1149F0363PCarbon steel, 0.45 in outside diameter, 0.06 in thick
AN960C10NAS1149C0363RCorrosion resistant steel version of the same size
AN960C416NAS1149C0463RCorrosion resistant steel, 1/4 in bolt size
AN960-1016NAS1149F1063P5/8 in carbon steel, cadmium plated

The pattern in these listings is consistent: the letter after NAS1149 tracks the material (F for the carbon steel parts above, C for corrosion resistant steel), the next two digits track the bolt size in sixteenths of an inch (03 for No. 10, 04 for 1/4 in, 10 for 5/8 in), the following two digits match the thickness in thousandths (63 against the 0.06 in listed), and the trailing letter tracks the finish. On the AN side, a C after AN960 means corrosion resistant steel. Treat this as a reading aid, not a substitute for the standard. The listings call these alternates, not certified equivalents, so for airworthiness work confirm against the current NAS1149 standard and your approved data before substituting. MS35338 lock washers are simpler: the dash number is the size, and NASM35338 kept the same part numbers when it replaced the MS sheet.

When lock washers are not allowed

Lock washers are the most restricted washer type in aerospace work. AC 43.13-1B paragraph 7-87 says they may be used with machine screws or bolts only where a self-locking or castellated nut is not applicable, and then lists where they should not be used at all:

  • Where frequent removal is required.
  • In areas subject to corrosion.
  • In areas exposed to airflow.
  • On primary structure, secondary structure or accessories where failure might result in damage or danger to the aircraft or personnel. The AC prints this as a caution.

When a lock washer is used, put a plain washer between it and the part so it does not gouge the surface; paragraph 7-86 repeats that plain washers go under lock washers. In practice that pushes structural joints toward self-locking nuts, castle nuts with cotter pins, safety wire or permanent fasteners such as the pins on the collars and lockbolts page. Nut options are on the nuts page.

Countersunk, tab, key and conical washers

Countersunk washers fit under a head with a large underhead radius. AC 43.13-1B says internal wrenching bolts need a heat treated countersunk NAS143C or MS20002C washer under the head, so the shank radius does not bear on the sharp edge of the hole, and a plain NAS143 or MS20002 under the nut. Leaving it out, or using a commercial washer, puts the bolt head radius on an edge and can crack the part.

Tab, key and cup washers lock a nut or bolt mechanically. A tab or key engages a slot or hole in the part, and a second tab is bent against a flat of the nut. MS key washers are made from corrosion resistant steel to AMS 5510 with 90, 180 or 270 degree locking positions, so the part number has to match the geometry of the mating part. Follow the drawing or maintenance manual on reuse; a bent tab is a work hardened tab.

Conical spring washers, often called Belleville washers, keep clamp load in a joint that relaxes from thermal cycling, gasket creep or a soft stack. They are usually source controlled or drawing specified rather than common MS numbers, so order by the manufacturer part number or drawing, and confirm load, deflection and material with the designer.

Materials, finishes and the trade-offs

Washer material is about the joint, not the washer. AC 43.13-1B recommends cadmium plated steel washers under bolt heads and nuts on aluminum alloy or magnesium structure to prevent corrosion, which is the galvanic logic behind most callouts: put a compatible, protected surface between the steel fastener and the light alloy. Corrosion resistant steel washers suit stainless and titanium joints and hot areas, 5052 aluminum NAS1197 washers suit aluminum structure where steel is not wanted, and resin fiber NAS549 washers serve as insulators. Cadmium is limited to 450 °F, as covered on the bolts and screws page.

Plain washers also serve as shims. AC 43.13-1B allows washers to make up grip, but caps the combined washer height at 1/8 in and uses washers to position castle nuts relative to the cotter pin hole. If a stack needs more than that, the bolt grip is wrong. Finish and passivation background is on the surface finishing page, and alloy notes on stainless and specialty steels.

Ordering checklist

  • Exact part number, including material, size, thickness and finish codes, and the controlling standard and revision. If the parts list shows an AN960 number, decide whether you will accept the NAS1149 alternate and record the approval.
  • For MS35338 numbers, whether NASM35338 certification is acceptable on the paperwork.
  • Manufacturer and lot traceability and a certificate of conformance; material certs where the program asks for them.
  • DFARS 252.225-7009 flow down for steel washers on DoD articles.
  • Packaging by size and lot. Thin and standard washers of the same size are easy to mix.

Counterfeit and quality risks, and how to receive washers

Washers are cheap, which is exactly why commercial washers get substituted. The usual problems are commercial flat washers in place of NAS1149 parts, wrong thickness, wrong material, missing or wrong plating, and lots mixed in one bag. On receipt, check outside and inside diameter and thickness on a sample against the standard, confirm magnetic response where it separates carbon steel from corrosion resistant steel, check the finish color and coverage, and match lot numbers to the CoC. For hardened countersunk washers under internal wrenching bolts, ask for the heat treat certification the standard requires. The receiving inspection guide and the counterfeit parts avoidance page cover the process.

How we source washers

We do not stock washers or publish prices. We source the exact NAS, NASM, MS or AN number through authorized and franchised distributors or the manufacturer, flow down the standard, finish and specialty metals requirements, and check the paperwork before shipment. Washers are usually best bought as part of a hardware kit with the bolts and nuts for each assembly, on the same PO as the machined parts. Older AN and MS numbers with thin supply are handled as obsolete parts. See the full range on the fasteners page.

Get washers and hardware kits quoted

Send the washer numbers or the full hardware list for an assembly. A person replies within one business day, with one quote and one point of contact, and the order ships with a certificate of conformance.

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Questions

Did NAS1149 replace AN960?

For numbering, yes. Industry guidance notes that washers identified as AN960 have been superseded by the NAS1149 numbering system, and distributors list specific AN960 numbers as alternates to specific NAS1149 numbers, for example AN960-10 against NAS1149F0363P. The listings call them alternates rather than certified equivalents, so on certificated or contract work confirm the cross-reference against the current NAS1149 standard and your approved data before you substitute.

When are lock washers not allowed on aircraft?

FAA AC 43.13-1B says not to use lock washers where frequent removal is required, in areas subject to corrosion, or in areas exposed to airflow. It also cautions that lock washers are not to be used on primary structure, secondary structure or accessories where failure could cause damage or danger to the aircraft or personnel. Where they are used, a plain washer goes between the lock washer and the part to prevent gouging.

Is MS35338 still current?

The MS35338 sheet was superseded by NASM35338 in 2002, and the notice states that the part numbers are identical. SAI Global lists NASM35338 in a 2014 edition as current. So an MS35338 part number can still be ordered, with NASM35338 as the controlling document. Remember that split lock washers carry the use restrictions in AC 43.13-1B, so the number being available does not mean it is acceptable in every joint.

Why does an internal wrenching bolt need a special washer?

Internal wrenching bolts have a large radius where the shank meets the head. AC 43.13-1B calls for a heat treated countersunk washer, NAS143C or MS20002C, under the head so that the radius seats in the washer's countersink instead of bearing on the sharp edge of the hole. A plain NAS143 or MS20002 washer goes under the nut. A commercial flat washer in that position can damage the bolt and the part.

What washer should go under a steel bolt on aluminum structure?

AC 43.13-1B recommends cadmium plated steel washers under bolt heads and nuts used on aluminum alloy or magnesium structure to prevent corrosion. The drawing or structural repair manual governs, and some programs use aluminum NAS1197 washers or a different protective finish. Check the cadmium temperature limit of 450 °F and any customer restrictions on cadmium before choosing.

How many washers can I stack to fix grip length?

AC 43.13-1B limits the combined height of washers used to compensate for grip to 1/8 in, which roughly allows going up one standard grip size. Washers are also used to position a castellated nut so the cotter pin hole lines up. If you need more than 1/8 in of washers, order the bolt with the correct grip instead of adding to the stack.

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