Reinforced cylinder head gasket BMW N54B30

The BMW engines covered

The BMW N54B30 is the 3.0 litre 24-valve twin-turbo inline six-cylinder petrol engine fitted to the 135i (E82 and E88) and the 1 M Coupé, the 335i (E90 to E93), the 535i (E60 and E61), the 740i (F01 and F02), the X6 xDrive35i (E71) and the Z4 sDrive35i and 35is (E89). Its bore is 84 mm with an 89.6 mm stroke, and all its versions receive the same original cylinder head gasket.

This original gasket carries the part number 11 12 7 557 265. BMW describes it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 85.3 mm and its thickness 1.22 mm. The repair size, 11 12 7 557 266, measures 1.52 mm, that is 0.30 mm more. It is sized for standard combustion pressure: when that pressure rises, clamping load drops, the cylinder head lifts, and coolant enters a cylinder.

Reinforced cylinder head gaskets

A cylinder head gasket does not create the seal on its own: it holds because the fasteners clamp the head against the block. A composite gasket is crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of the MLX gaskets withstands 517 N/mm², 5 times more, and springs back at every combustion. The O-Ring, for its part, separates the functions: its stainless ring alone seals combustion, the body handles the water and oil circuits, and nothing has to be machined in the block.

This page brings together the part numbers from two racing manufacturers, Athena and Cometic, in two constructions: O-Ring with stainless ring and MLX with combustion chamber stopper. Gasket bores range from 85 to 86 mm, covering the original block as well as the rebored block, and thicknesses from 0.81 to 1.85 mm, enough to adjust the compression ratio without touching the pistons or the cylinder head. 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

Gasket volume: the room the gasket leaves free above each cylinder. It adds to the combustion chamber, so the bigger it is, the lower the compression ratio goes. These values are only compared between gaskets of the same bore.

Preparation and fitting

Before assembly, check with a straight edge that the cylinder head and the block are flat, and have them skimmed if needed. No sealant to plan for: Athena and Cometic both supply these gaskets already coated.

Technical guide

The two constructions offered

Athena O-Ring / Cut Ring A steel sheet clad with asbestos-free aramid fibre, with a stainless steel compression ring around each of the cylinders. The ring narrows the clamping force onto a small diameter, right where sealing takes place, and a silicone bead guards the water and oil passages. It is the construction that resists the highest pressures, and the only one that matches a cylinder head already prepared with a compression wire.

Cometic MLX Several stamped and riveted stainless steel layers, with a folded centre layer forming a stopper around the combustion chamber, and a high-temperature coating. Local clamping increases without bore distortion growing. It is the most resistant of the all-steel gaskets, to be chosen for high boost levels, in 85 mm only.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 86 mm, the Cometic MLX in 85 mm.

Thickness. The original gasket is 1.22 mm thick. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If the chosen thickness is thinner than the original, measure with a dial gauge how far the pistons protrude above the block: this measurement gives the remaining clearance between piston and cylinder head.

The two brands

	An Italian manufacturer of engine gaskets and components, Athena S.p.A. is located in Alonte, in the Veneto region, home to its head office and main factory. The group also manufactures in Spain, the United States, Brazil and India. Car manufacturers fit its multi-layer gaskets as original equipment, and its racing range is fully designed and produced in Italy.
	An American company founded in 1989 and based in Concord, Ohio, Cometic Gasket, Inc. designs engine gaskets and sealing solutions. It has made a speciality of the gasket produced on demand, with the bore and thickness required. All of its production is carried out in the United States, from American materials.

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 85.3 mm bore of the original gasket 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: 11 12 7 557 265 in standard size, 11 12 7 557 266 in repair size.

Important: the N55B30, which succeeded the N54B30 from 2010 on the 135i, 335i and X6 xDrive35i, receives another original cylinder head gasket, part numbers 11 12 7 599 212 and 11 12 7 599 213. These vehicles with the N55B30 engine are not covered by the part numbers on this page.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, the reinforced one keeps holding: under increased combustion pressure, it keeps its shape without being crushed.

With an added turbo, a supercharger or a raised compression ratio, power climbs and the head gasket becomes the most fragile point of the top end. Going for a reinforced gasket is then nothing like a comfort: it is a safeguard, the one that spares you the failure and the cylinder head removal that goes with it.