Cometic MLS Exhaust Manifold Gasket | Nissan RB20 / RB25 | .030" | 1.575" x 1.340" Port | C4177-030
Cometic MLS Exhaust Manifold Gasket – Nissan RB20 / RB25 | .030" | 1.575" x 1.340" Port | C4177-030
Fits Nissan RB20 and RB25 engines — not for RB26DET (Skyline GT-R). The RB26DET uses a different exhaust port configuration and requires a separate Cometic part number. Covers RB20DET, RB25DE, and RB25DET applications. Port size is 1.575" x 1.340" — confirm this matches your head and manifold port dimensions before ordering, particularly on ported or modified heads. Ships fast across Canada and the US.
Cometic C4177-030 Multi-Layer Steel (MLS) exhaust manifold gasket for the Nissan RB20 and RB25 inline-6 engine family. .030-inch compressed thickness, 1.575" x 1.340" port opening. Seals the exhaust manifold or header flange to the RB20/25 cylinder head without sealants. Resists heat cycling, exhaust pressure pulses, and thermal expansion. 0.4 lbs. Ships fast across Canada and the US.
MLS Construction — Why It Matters for RB Exhaust Applications
Cometic’s MLS (Multi-Layer Steel) exhaust manifold gaskets use multiple embossed steel layers that compress under torque and maintain spring pressure against the sealing faces through repeated heat cycles. The key advantages over OEM composite exhaust manifold gaskets on the RB20/25:
- Won’t burn through: OEM composite exhaust gaskets on the RB series are a known failure point on modified engines running elevated boost and exhaust temperatures. MLS steel construction is impervious to the temperature ranges seen in performance RB applications.
- Maintains torque set: Composite gaskets crush and relax over time, allowing manifold studs to loosen and the gasket sealing surface to deteriorate. MLS gaskets maintain their spring-back compression over the service life of the engine.
- No sealants required: MLS exhaust gaskets are installed dry — no exhaust sealant, copper spray, or RTV required. Sealants can actually interfere with MLS gasket function by filling the embossed sealing beads.
- Withstands mixed materials: The RB20/25 aluminium cylinder head and cast iron or steel exhaust manifold/header expand at different rates under heat. The MLS layers accommodate this differential expansion while maintaining a positive seal.
RB20 and RB25 — Shared Exhaust Port Dimensions
The Cometic C4177-030 covers both the RB20 and RB25 engine families, which share the same exhaust port flange dimensions at the cylinder head face. Applications include:
- RB20DET: 2.0L twin-cam turbo — Nissan Skyline R32 GTS-t, R33 GTS-t
- RB25DE: 2.5L twin-cam naturally-aspirated — R33 Skyline, Stagea WC34
- RB25DET: 2.5L twin-cam turbo — R33 Skyline GTS-t, R34 Skyline 25GT-t, Stagea WC34
Not for RB26DET (R32/R33/R34 Skyline GT-R) — the RB26 uses a different exhaust port size and flange configuration. Order the correct Cometic part number for your specific engine.
Port Dimensions — 1.575" x 1.340"
The C4177-030 has a 1.575" wide x 1.340" tall exhaust port opening — matching the OEM RB20/25 exhaust port size at the cylinder head face. For stock cylinder heads and standard aftermarket headers designed for the OEM port size, these dimensions are correct. If your cylinder head has been port-matched to a larger diameter manifold or header, verify that the gasket opening matches or exceeds your ported port diameter before installing. Installing a gasket with a smaller opening than the ported head creates a step that can restrict exhaust flow and create a turbulence point.
RB20 and RB25 only — not for RB26DET. No sealant required. Install on clean, flat sealing surfaces. Replace manifold studs if corroded or damaged. Torque to manifold manufacturer’s specification.
Specifications
- Brand: Cometic Gasket
- Part Number: C4177-030
- Type: MLS exhaust manifold gasket
- Thickness: .030" (0.762mm) compressed
- Port size: 1.575" x 1.340"
- Application: Nissan RB20DET, RB25DE, RB25DET
- Not for: RB26DET
- Sealant required: No
- Weight: 0.4 lbs