The right intake manifold vacuum port fitting is the one that matches your manifold connection, the hose or line attached to it, and the circuit it serves. A fitting described as “3/8 inch” might refer to a pipe thread, a hose barb, or a tube connection. Those descriptions are not interchangeable.
For a stock vehicle repair, start with the vehicle manufacturer’s parts catalog and vacuum-routing diagram. For an aftermarket intake, start with that manifold’s instructions. The options below cover several distinct applications; they are a fitment shortlist rather than a universal ranking.
Quick choices by application
- Plastic LS truck intake with the specified rear push-lock port and a 1/2-inch hose connection: investigate ICT Billet 551927-BARB.
- The same specified LS port, but an approved -6 AN hose-end arrangement: investigate ICT Billet 551927-6AN.
- A threaded manifold needing two smaller threaded outlets: JEGS 16005 has a 3/8-inch NPT male inlet and two 1/8-inch NPT female ports.
- A stock GM repair calling for part 12559760: use the GM connector after checking the vehicle application.
Do not buy from this list until the manifold-side connection is identified. Similar engine displacement, a familiar brand name, or a photograph of a matching-looking nipple does not establish fit.
Four fittings worth checking
ICT Billet 551927-BARB: for its specified LS push-lock port
ICT Billet identifies this fitting for plastic LS truck intake manifolds with a push-lock rear brake-booster port. The outlet is a 1/2-inch hose barb; the manifold-side attachment is the specified push-lock connection, not a 1/2-inch NPT thread. The manufacturer lists 6061-T6 billet aluminum and supplies the fitting only. See the 551927-BARB manufacturer specification.
This is a candidate when the existing manifold and required hose connection match that description. Confirm the manifold’s identity, retaining arrangement, seal requirements, and hose specification with ICT Billet or the service provider. It is not a reason to replace a differently sized original booster hose, and the listing does not make it an adapter for every LS intake or aftermarket manifold.
ICT Billet 551927-6AN: a different outlet connection
The 551927-6AN manufacturer page describes an LS truck intake brake-booster port adapter with a -6 AN male flare outlet. Select it only when the complete, application-approved hose assembly uses the matching connection.
An AN flare connection is not a hose barb. A plain rubber hose cannot be treated as an AN hose end, and a threaded part that happens to screw on is not necessarily a compatible sealing connection. Holley’s AN hardline guidance identifies the 37-degree flare used in that system. Have the hose and fitting supplier confirm the complete assembly, including its vacuum suitability.
JEGS 16005: threaded adapter with two small outlets
JEGS 16005 is a nickel-plated brass adapter with a 3/8-inch NPT male connection and dual 1/8-inch NPT female ports. JEGS describes its low-profile form as useful for clearance beneath carburetors with a rear fuel bowl. See the JEGS 16005 product details.
The two outlet measurements are thread sizes, not supplied hose-barb sizes. Any additional adapters must be specified separately for the intended hoses and circuits. Two available ports do not establish that a brake booster and PCV line should share this fitting. Also verify clearance with the actual carburetor, linkage, fuel lines, and installed hose bends; a low-profile description does not guarantee room on your engine.
GM Genuine Parts 12559760: OE application first
GM identifies 12559760 as a power-brake-booster vacuum-hose connector and lists it as a single component. The official Chevrolet parts page includes a vehicle-fitment check. Use the VIN, engine, and relevant build information to confirm it before ordering.
This is the most straightforward route when the service information specifies that exact OE part. It is not a universal replacement for every plastic manifold nipple, and the product page should not be used to guess an unlisted thread or hose dimension. A damaged seal, retaining feature, hose, or check valve may need separate attention.
GM Genuine Parts 12559760 Power Brake Booster Vacuum Hose Connector, 1 connector
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Thread size and hose size: read both ends
Write a fitting description as two separate connections. For example, “3/8-inch NPT male to 1/2-inch hose barb” describes one threaded end and one hose end. Neither measurement substitutes for the other. A hose-barb size refers to the intended hose bore; the peaks of the barb can measure larger.
NPT is a tapered pipe-thread system. Its nominal size is not a direct measurement of the thread’s outside diameter. Parker’s thread and tube size chart lists 1/8-inch NPT as 27 threads per inch and 3/8-inch NPT as 18 threads per inch. That distinction helps identify a specification; it is not permission to trial-fit unknown parts.
Do not substitute BSP, metric threads, AN flare, or O-ring-boss connections because their diameters look close. Likewise, an LS push-lock adapter relies on its intended port geometry and retention. If the connection cannot be positively identified, send the manufacturer the manifold part number and clear photos before buying.
Brake booster, PCV, and other vacuum circuits
Vacuum ports perform different jobs. A brake-booster supply, a PCV connection, a distributor’s timed-vacuum connection, and a sensor reference line are not interchangeable merely because they all carry air. Holley’s carburetor instructions distinguish full-manifold, timed-spark, PCV, and power-brake connections and direct owners to the appropriate service manual for routing questions.
Preserve the vehicle’s required check valves, restrictions, hose specifications, and emissions connections. Do not tee into the booster supply, cap a required line, or reroute PCV or evaporative-emissions plumbing simply to make a new fitting usable. A multi-port distribution block also needs application-specific approval; the number of ports is not evidence of adequate flow or correct circuit behavior.
If brake assist has changed, the pedal feels unusually hard, or a booster hose or fitting is damaged, stop driving and have the vehicle inspected. A shopping guide cannot establish that the braking system is safe.
A practical pre-purchase checklist
- Identify the manifold: record the vehicle application or the aftermarket maker and exact part number.
- Identify the circuit: use the routing diagram to establish where the hose belongs and what components must remain in that path.
- Specify both connections: thread standard, nominal size, male/female ends, or the exact quick-connect design, plus the required hose bore or hose-end type.
- Check the complete assembly: seals, retainers, clamps, check valves, and any additional adapters may be separate parts.
- Check packaging and clearance: a single fitting is not a hose kit; account for fitting height, hose bend radius, heat, and moving parts.
- Get installation requirements: ask for the correct sealant, seal preparation, and tightening method for that fitting and manifold material.
Installation and leak concerns
There is no universal torque value or “number of turns” that safely applies to all these fittings. Follow the manifold and fitting makers’ instructions; a qualified technician should resolve conflicting guidance. Forcing a tapered metal fitting into the wrong port can damage the manifold. Adding sealant does not correct incompatible threads or a damaged quick-connect seal.
After an authorized repair, the technician should verify hose routing, secure retention, leak-free operation, and proper brake-assist function where relevant. Avoid improvised leak tests involving flammable sprays. Household plumbing fittings, HVAC service manifolds, coolant-hose adapters, and generic vacuum caps are not substitutes merely because an online listing uses the word “vacuum.”
Bottom line
Choose by the exact connection and documented application. The ICT Billet options address specific LS push-lock ports with different outlets; JEGS 16005 addresses a threaded adapter requirement; GM 12559760 addresses specified OE repairs. When fit, circuit routing, or installation requirements remain uncertain, get that answer before ordering.
