How to Read a D38999 Part Number: MIL-DTL-38999 Decoded
D38999/20WJ35PA looks like noise until you know the pattern. Every character is a decision about how the connector mounts, what it's plated with, how big it is, how many contacts it carries and which way it's keyed. Change one and you have a different part.
/20 wall-mount flange receptacle, Series III · W aluminum shell, olive-drab cadmium over nickel · J shell size 25 · 35 insert arrangement 25-35, which carries 128 size 22D contacts · P pin contacts · A alternate key position A.
The six fields
A Series III or IV number reads D38999 / slash sheet · class · shell size · insert arrangement · contact type · key.
| Field | Example | What it tells you |
|---|---|---|
| Slash sheet | /20 | Connector style and mounting: plug, wall-mount, jam-nut, box-mount, hermetic. |
| Class | W | Shell material and plating. This is also where availability tends to break down. |
| Shell size | J | A letter code for the shell size, A (9) through J (25). |
| Insert arrangement | 35 | The contact layout inside that shell, standardized in MIL-STD-1560. |
| Contact type | P | P pins, S sockets; hermetic receptacles use their own letters. |
| Key | A | N for normal, A–E for alternate rotations so look-alike connectors can't cross-mate. |
Slash sheets: style and mounting
Series III uses a threaded, self-locking coupling and a scoop-proof design. Series IV uses a breech-lock coupling. The styles pair up:
| Style | Series III | Series IV |
|---|---|---|
| Wall-mount flange receptacle | /20 | /40 |
| Box-mount receptacle, hermetic | /21 | /41 |
| Jam-nut receptacle, hermetic | /23 | /43 |
| Jam-nut receptacle | /24 | /44 |
| Solder-mount receptacle, hermetic | /25 | /45 |
| Plug | /26 | /46 |
| Weld-mount receptacle, hermetic | /27 | /48 |
Series I (bayonet, scoop-proof) and Series II (bayonet, low silhouette) predate the D38999 slash-sheet scheme and are still widely ordered under their MS numbers, such as MS27466 (Series I wall-mount receptacle) and MS27473 (Series II plug). Their shell sizes are written as numbers (8 through 25) instead of letters, and their finish letter sits in a different position. If a drawing mixes the two schemes, call it out on the RFQ.
Class: shell material and finish
| Class | Shell | Finish |
|---|---|---|
| W | Aluminum | Olive-drab cadmium over nickel |
| F | Aluminum | Electroless nickel |
| Z | Aluminum | Black zinc-nickel |
| T | Aluminum | Nickel-fluorocarbon polymer |
| J | Composite | Olive-drab cadmium |
| M | Composite | Nickel |
| K | Stainless steel | Passivated, firewall rated |
| Y, N | Stainless steel | Hermetic: passivated (Y) or electro-deposited nickel (N) |
W is the classic military finish and still the most requested. Pressure to move away from cadmium has made Z and T common cadmium-free choices on newer designs. Changing finish changes the part number and usually needs engineering sign-off, so treat a finish swap as a substitution, not a formality.
Shell size letters
| Letter | A | B | C | D | E | F | G | H | J |
|---|---|---|---|---|---|---|---|---|---|
| Shell size | 9 | 11 | 13 | 15 | 17 | 19 | 21 | 23 | 25 |
Insert arrangement
The number after the shell letter is the contact layout. Read it together with the shell size: the "35" in a B-shell (11-35) carries 13 size 22D contacts, while the "35" in a J-shell (25-35) carries 128. Arrangements mix contact sizes 22D, 20, 16, 12 and larger, and some include coax, twinax or fiber cavities. The layouts are standardized in MIL-STD-1560, so the same arrangement number means the same pattern from every qualified manufacturer.
Contacts and keying
P and S are pin and socket. Hermetic receptacles use their own contact codes because the contacts are fused into the insert. Replacement crimp contacts are ordered separately under M39029: /58 pins and /56 sockets for Series III.
The last letter is the key. N is normal; A through E rotate the keyways so that identical connectors side by side on a harness can't be plugged into the wrong mate. A plug and its receptacle must carry the same key letter. Dropping it from an RFQ is one of the most common causes of a wrong shipment.
Mil part number or manufacturer number?
Amphenol, Glenair, ITT Cannon, Souriau and TE all sell the same configurations under their own catalog numbers, and those numbers often appear on drawings and BOMs. A D38999 number buys a qualified part built to the specification. A manufacturer number buys that manufacturer's product, which may or may not be the qualified version. When a drawing calls the mil number, quote the mil number; when it calls a commercial number, a cross to D38999 needs your engineer's approval, not ours.
Before you send the RFQ
- Send the full part number, including the key letter, exactly as the drawing shows it.
- Say whether contacts, backshells and dust caps are in scope so the kit quotes as one job.
- State date-code limits and any lot requirements, especially for hermetic receptacles.
- Flag finish flexibility. If an F or Z finish is acceptable where W is short, say so up front.
- List the paperwork you need: certificate of conformance, traceability, test reports.
Browse configurations on our MIL-DTL-38999 connector page, or send the part number and we'll answer with stock, alternates with the same intermateability, and the qualification status of each. We never substitute quietly.
Common questions
What does the W mean in a D38999 part number?
W is the class code for an aluminum shell with olive-drab cadmium plating over nickel, the traditional military finish. F is electroless nickel, Z is black zinc-nickel and T is nickel-fluorocarbon polymer.
How do I find the shell size in a D38999 part number?
For Series III and IV it is the letter right after the class: A = 9, B = 11, C = 13, D = 15, E = 17, F = 19, G = 21, H = 23, J = 25. In D38999/20WJ35PA the J means shell size 25.
What is the last letter of a D38999 part number?
The key (polarization) position. N is normal; A through E are alternate rotations that stop look-alike connectors from cross-mating. A plug and receptacle must have the same key letter to mate.
Can I substitute a different finish class?
Only with engineering approval. A different class is a different part number with different plating, corrosion and conductivity properties. Many buyers accept F or Z where W is short, but that decision belongs to the drawing owner.