Reinforced cylinder head gasket Nissan RB25DET 2.5L 24V Turbo

The Nissan engines covered

The Nissan RB25DET petrol engine, a turbocharged inline six-cylinder of 2.5 litres with 24 valves, powers among others the Skyline (R33 and R34), the Laurel (C34 and C35), the Stagea and the Cedric listed below. A single cylinder head covers all six cylinders: only one gasket is needed per engine.

The original part number from Nissan is 11044-75T02. Nissan publishes neither the bore, nor the thickness, nor the construction of the original gasket. That gasket is sized for standard pressure: once it rises, the clamping load relaxes, the head lifts and coolant reaches a cylinder.

Reinforced cylinder head gaskets

Sealing does not depend on the gasket alone: the fasteners that clamp the head to the block count just as much. A composite gasket settles permanently beyond 100 N/mm²; the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more, and regains its shape at every combustion. This also applies to the fasteners of this inline six.

This page gathers the part numbers of Cometic, an American maker of competition gaskets, in a single construction: MLS, multilayer steel. The gaskets come in three bores, 86, 87 and 88 mm, and in thicknesses from 0.69 to 3.56 mm (1.52 mm only in 88 mm), enough to adjust the compression ratio without touching the pistons or the cylinder head. As a rule, the gasket bore chosen is at least 0.5 mm larger than the actual cylinder bore. Each part number is sold individually, one gasket per engine.

Technical specifications

A gasket's volume is the space it leaves open above each cylinder. It adds to the volume of the combustion chamber and lowers the compression ratio by the same proportion. Compare only gaskets of the same bore. On this turbocharged six, this volume depends on the bore and thickness chosen.

Preparation and fitting

Before fitting, check with a straightedge that the gasket faces of the head and the block are flat, and have the warped one resurfaced. Apply no sealant: Cometic gaskets are supplied coated. On this inline six, run this check on the block as well as on the head.

Technical guide

The construction offered

MLS Cometic A stack of stamped stainless steel sheets assembled with rivets. The outer sheets have active beads around each cylinder and each fluid passage. Moderate clamping is enough to seal and the bore distorts little. A versatile construction, from the tuned naturally aspirated engine to moderate forced induction. For this turbocharged six, it is offered in three bores.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in an 86 mm bore (C4317), an 87 mm bore (C4318) and an 88 mm bore (H2025SP1060S, single part number).

Thickness. Nissan publishes no thickness for the factory gasket of this turbocharged inline six. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If you choose a thickness below the standard one, first measure with a dial gauge by how much the pistons protrude above the block face: this figure gives the clearance remaining between piston and head. On this inline six, take this measurement on the bare block, before any assembly.

The brand

Cometic Gasket, Inc. is an American company established in Concord, Ohio, since 1989. It designs gaskets and sealing solutions for engines, and its reputation comes from the made-to-measure gasket, supplied at the bore and thickness requested. All production remains in the United States, from American raw materials. Its gaskets are offered here for this Nissan turbocharged six, in three bores.

Advantages over the original gasket

When a reinforced gasket becomes necessary

• Adding forced induction, or raising boost on an already turbocharged engine
• Cylinder rebored beyond what the bore size of this engine's factory gasket, a figure Nissan does not communicate covers
• Looking for a compression ratio different from standard, higher or lower
• Engine that has already lost its gasket, or is consuming coolant
• Competition use, where the gasket goes through repeated temperature rises

Vehicle compatibility

OEM part number: 11044-75T02.

Important: the part numbers on this page match the Nissan RB25DET engine listed above: note your vehicle's engine code, then pick the gasket bore (86, 87 or 88 mm) suited to your cylinders before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, the reinforced gasket replaces it: under higher combustion pressure, it holds its shape instead of being crushed. On this inline six, the thickness is chosen from the measurement taken.

With a turbocharger, a supercharger or a raised compression ratio, the cylinder head works harder and the gasket becomes the weak link of the top end. The reinforced gasket rules out a gas or coolant leak and the head removal that goes with it, with the vehicle off the road. This is the case for builds of this turbocharged six, already boosted as built.