D38999 Series III vs MS27467 Series I: Key Differences
MIL-DTL-38999 is one specification, but it covers four physically different connector families. Two of them show up on Los Angeles harness drawings more than any others: Series I, ordered by MS27467-style numbers, and Series III, ordered by D38999/2x slash-sheet numbers. They look similar and share the same insert arrangements, and they do not mate. This guide explains how to tell the series apart, what each one is specified for, and how to read the two part-number formats so the right connector gets quoted the first time.
The four series of MIL-DTL-38999 at a glance
Series I: bayonet, scoop-proof (MS27466, MS27467, MS27468)
Series I is the original 38999 design. It uses a three-point bayonet coupling with a rotating coupling ring and an audible, visual and tactile lock. The plug shell is long enough that the receptacle pins cannot be damaged by a misaligned plug during mating, which is what “scoop-proof” means. The plug also carries EMI grounding fingers as standard. The common shell styles are MS27466 (wall-mount receptacle), MS27467 (straight plug) and MS27468 (jam-nut receptacle), with MS27496 and MS27505 covering box-mount receptacles and MS27656 covering back-panel wall mounting. Amphenol LJT is the commercial equivalent. Shell sizes run 9 through 25.
Series II: bayonet, low profile, not scoop-proof (MS27472, MS27473, MS27474)
Series II keeps the bayonet coupling but shortens the shells to save weight and space. That shorter plug is the trade-off: Series II is not scoop-proof. Shell styles include MS27472 and MS27497 (wall-mount receptacles), MS27499 and MS27508 (box-mount receptacles), MS27473 (straight plug), MS27484 (straight plug with grounding fingers), MS27474 (jam-nut receptacle) and MS27500 (90-degree plug). Amphenol’s JT family is the commercial equivalent. Shell sizes run 8 through 24.
Series III: triple-start threaded coupling, scoop-proof (D38999/20, /24, /26)
Series III replaced the bayonet with a self-locking triple-start threaded coupling that fully mates in less than one turn of the coupling nut and includes an anti-decoupling ratchet for high-vibration service. It is scoop-proof, and the plug carries EMI grounding fingers that engage the receptacle shell before the contacts touch. The everyday slash sheets are D38999/20 (wall-mount receptacle), D38999/24 (jam-nut receptacle) and D38999/26 (straight plug). Hermetic receptacles are /21, /23, /25 and /27, and /29 through /31 are lanyard-release plugs. Protective covers for Series III are D38999/32 (receptacle) and D38999/33 (plug).
Series IV: breech-lock coupling (D38999/40 through /47)
Series IV uses a quarter-turn breech-lock coupling: three locking lugs engage with a short rotation and no threads to cross. It is scoop-proof, quick to mate in tight or blind locations, and intended for very high vibration. Slash sheets are D38999/40 (wall-mount receptacle), /42 (box-mount receptacle), /44 (jam-nut receptacle) and /46 and /47 (plugs). It is usually build-to-order rather than stocked.
Comparison table
| Feature | Series I | Series II | Series III | Series IV |
|---|---|---|---|---|
| Typical part numbers | MS27466, MS27467, MS27468 | MS27472, MS27473, MS27474, MS27497, MS27499 | D38999/20, /24, /26 | D38999/40, /44, /46 |
| Coupling | Bayonet, three-point | Bayonet, three-point | Triple-start thread, self-locking | Breech-lock, quarter turn |
| Scoop-proof | Yes | No | Yes | Yes |
| EMI grounding fingers | Standard on plugs | Optional (MS27484 plug) | Standard on plugs | Yes |
| Rated mating cycles | 500 | 500 | 1,500 | High-cycle; confirm on slash sheet |
| Shell sizes | 9 to 25 | 8 to 24 | 9 to 25 (letters A to J) | 9 to 25 (letters A to J) |
| Rear accessory thread family | Shared with Series II | Shared with Series I | Series III/IV family (M85049 for 38999 III) | Same as Series III |
Class and finish letters
The class letter is the single most common source of wrong quotes, because Series I/II and Series III use different letter schemes.
On Series III (D38999/2x) the class letter follows the slash number and describes shell material and plating in one character. The ones you will see most:
- W: aluminum, olive drab cadmium over nickel. The traditional 500-hour finish and still the most common on legacy military drawings.
- F: aluminum, electroless nickel. Conductive, cadmium-free, the usual choice where cadmium is restricted.
- Z: aluminum, black zinc-nickel. The cadmium-free replacement for W with comparable corrosion resistance, increasingly specified on new programs.
- K: stainless steel, passivated, firewall rated. Heavy, expensive and mandatory where the drawing says firewall.
- J and M: composite shells with olive drab cadmium or electroless nickel respectively. Hermetic receptacles use their own class letters.
On Series I and II MS numbers the “class” and the “finish” are two separate positions. The class letter (E for environmental, T for environmental with serrated rear threads and no rear accessories, P for potting, Y for hermetic) describes the rear termination style, and a separate finish letter (A for cadmium over nickel, B for olive drab cadmium over nickel, F for electroless nickel, C for anodic coating) describes plating. A Series I T and a Series III W are not the same thing.
Part-number anatomy
Series III example: D38999/26WB35SN
- D38999/26: straight plug, Series III.
- W: class, aluminum shell with olive drab cadmium over nickel.
- B: shell size 11 (A=9, B=11, C=13, D=15, E=17, F=19, G=21, H=23, J=25).
- 35: insert arrangement per MIL-STD-1560; in shell size 11 this is 13 size-22D contacts.
- S: socket contacts supplied (P for pins; A and B indicate pin or socket inserts supplied without contacts).
- N: normal polarization (A through E are the alternate key positions).
Series I example: MS27467T15B35S
- MS27467: straight plug, Series I.
- T: class, environmental with serrated rear threads and no rear accessories.
- 15: shell size, written as a number, not a letter.
- B: finish, olive drab cadmium over nickel.
- 35: insert arrangement; in shell size 15 this is 37 size-22D contacts.
- S: socket contacts. A trailing A, B, C or D would indicate an alternate key position; no letter means normal.
Both are “-35” arrangements in olive drab cadmium with size-22D contacts, which is why they get confused. The shell-size position, letter versus number, is the fastest way to tell them apart.
Interchangeability warning
Series I and Series III do not mate with each other. The coupling mechanisms are physically different: a bayonet plug cannot thread onto a Series III receptacle, and a threaded Series III plug cannot lock onto a bayonet receptacle. Series II does not mate with either of them, and Series IV is its own interface. Within a series, a plug and receptacle of the same shell size, same insert arrangement and same polarization will mate regardless of class letter or manufacturer.
What does carry across the series is the contact system. Series I, II and III all use MIL-STD-1560 insert arrangements and M39029/56 pin and M39029/58 socket crimp contacts in sizes 22D through 12, so the contact and tooling call-outs on a drawing will not tell you which series you are looking at. Rear accessories are a different story: Series I and II share an accessory thread family with each other, while Series III and IV use a different one, so an M85049 backshell ordered for a D38999/26 plug will not fit an MS27467 of the same shell size.
Which one is on my drawing? A checklist
- Does the part number start with “D38999/” followed by 20 through 31? Series III. If it is /40 through /47, Series IV.
- Does it start with “MS27” followed by 466 through 471, 496, 505 or 656? Series I. MS27472 through 484, 497, 499, 500 or 508? Series II.
- Is the shell size a letter (A through J)? Series III or IV. A two-digit number? Series I or II.
- Does the class letter sit directly after the slash number with no separate finish letter? Series III. Is there a class letter and then a finish letter with the shell size between them? Series I or II.
- Does the drawing note call out a bayonet coupling, an LJT or JT commercial equivalent, or an MS27501/MS27502 dust cap? Series I.
- Does the note call out a D38999/32 or /33 cover, a triple-start thread, 1,500 cycles or an M85049/38 strain relief? Series III.
Which series to specify
Series III is the default for new designs and most current-platform repairs. Series I stays in production for the large legacy installed base built around it. Series II is chosen where weight and envelope matter more than scoop-proofing, and Series IV where blind, quarter-turn mating under severe vibration is required.
Newcastle Industries stocks D38999 Series III and MS27467-series Series I connectors from Amphenol, Glenair and Conesys/Aero-Electric in Los Angeles, along with matching M85049 backshells, protective covers and M39029 contacts. Send the full part number from the drawing and we will confirm the series and quote from stock.