Connector seals and EMI gaskets for electrical connector makers

What a connector maker buys, part by part

The table follows a circular connector from its mating face to the panel it mounts on. The material column gives what the MIL-DTL-38999 text names; a commercial connector's drawing names its own compound, and the drawing governs.

What MIL-DTL-38999 says about its seals

MIL-DTL-38999 is the Department of Defense detail specification for miniature, high-density circular connectors with bayonet, threaded or breech coupling, made either environment resistant, with crimp contacts that can be removed, or hermetic, with fixed solder contacts [S45][S46]. It covers four series, and Series I, II, III and IV are neither interchangeable nor intermateable [S46]. DLA ASSIST lists Revision N with Amendment 2, dated 27 July 2026, as the current issue [S45]. The paragraphs summarized on this page are from Revision M with Amendment 2, the latest full text EverySpec hosts, so check them against the revision your drawing cites [S46].

A resilient interfacial seal is bonded across the face of every pin insert, and the raised barrier around each pin seats in the lead-in chamfer of the matching socket hole [S46]. Mated, that seal isolates each contact from its neighbors and from the shell [S46]. The grommet is the resilient back section of the insert that seals around the wire; a cavity left unwired takes a sealing plug instead, and the unit pack carries spare plugs [S46]. Receptacles in Series I, II and III carry a peripheral seal, Series IV plugs use an O-ring for it, and every jam-nut receptacle comes with an O-ring to MIL-R-25988/3 [S46].

For the gaskets, grommets and interfacial seals, the material paragraphs name silicone or fluorosilicone elastomer for every aluminum, steel and composite class [S46]. In the indirect lightning strike test for Series III, damage to an elastomer sealing member, or hardening that affects its sealing, counts as a failure [S46]. At inspection, a gasket that is missing, twisted, buckled or kinked is cause for rejection [S46].

Where the shield runs, and where an EMI gasket comes in

On the connector itself the shield is metal. Conductive shell finishes carry continuity from one mated shell to the other, and the plug's spring fingers, which the specification also calls EMI bands, grounding springs or grounding fingers, bond the two shells together [S46]. The specification tests shell-to-shell conductivity, and it measures how well mated connectors fitted with EMI backshells shield from 100 MHz to 10 GHz; class C and finish C are exempt from both tests [S46].

The elastomer joins the shield path at the mounting. Where a receptacle flange bolts to a shielded enclosure wall, the joint is a seam like any cover or door, and a conductive gasket keeps it electrically continuous; the EMI gasket guide explains when a seam needs one. MIL-DTL-83528 gives connector flange mount gaskets their own specification sheet, /4, and the manufacturers' molded shapes include connector-flange gaskets and jam-nut seals [S3][S15].

EMI shielding gaskets for connector flanges

An EMI gasket for a connector flange comes in three common forms: a flat gasket cut from conductive silicone or fluorosilicone sheet with holes for the fasteners and the connector body, a conductive O-ring in a groove, and a molded flange gasket to MIL-DTL-83528 [S3][S15]. Conquest Seal waterjet-cuts conductive silicone and fluorosilicone sheet to your drawing in Orange, California, using sheet from a QPL-83528 listed maker when the drawing calls out MIL-DTL-83528. Molded flange gaskets, conductive O-rings, extrusions, and spliced and vulcanized rings are supplied from the manufacturers.

The filler your drawing names sets the conductivity, the weight and how the gasket behaves against an aluminum or plated flange. The conductive elastomers page lists the fillers with the makers' published figures, and the galvanic compatibility guide covers the pairing of filler and flange finish. The guides below go deeper, one question each.

1. What is an EMI gasket? EMI, RFI, EMC and grounding
2. MIL-DTL-83528 explained: type letters, sheets and the QPL
3. EMI gasket design: groove, deflection and closure force
4. Galvanic compatibility of fillers and flange finishes
5. Shielding effectiveness testing
6. EMI gasket forms compared
7. EMI gasket failure modes
8. Cutting conductive silicone: waterjet, die and knife
9. EMI gaskets with adhesive backing

Silicone and fluorosilicone on connector drawings

MIL-DTL-38999 allows either polymer for its seals, so the connector maker's drawing picks one [S46]. The M25988 documents cover fluorosilicone that resists oil and fuel, which is why a jam-nut O-ring or a seal near fuel or hydraulic fluid is often fluorosilicone. The fluorosilicone page and the silicone page give each family's published range, and the M25988 page decodes the four slash sheets.

What to send for a quote

The more of this the request carries, the fewer questions come back.

1. The connector specification and slash sheet, or your own drawing number and revision.
2. The part: interfacial seal, grommet, O-ring, peripheral seal or flange gasket, with its drawing or a DXF.
3. The material callout: silicone or fluorosilicone and its hardness, the filler and binder or MIL-DTL-83528 type letter for a conductive part, or the M25988 slash sheet and dash number.
4. The quantity, with any blanket or release schedule.
5. The paperwork the order needs: the certificate of conformance, the manufacturer's certificate, or an AS9102 first article report.

Please don't send a drawing marked ITAR, EAR, CUI or export-controlled. For those parts, call (714) 632-9830. Send the rest through request a quote, and a person replies within one business day. A certificate of conformance ships with every order, naming the part number, the specification, the manufacturer, the lot, the quantity and the manufacturer's cure date. Conquest Seal is AS9100D certified, with a scope that includes distribution, and the compliance documents page lists what a supplier file usually needs.

The specification text is summarized in our own words with its paragraphs cited; the revision your drawing cites, and the drawing itself, govern.