Reinforced cylinder head gasket BMW S55B30

The BMW engines covered

The BMW S55B30 is the 24-valve twin-turbo inline six-cylinder petrol engine of 3.0 litres fitted to the M2 Competition and M2 CS (F87), the M3 (F80) and the M4 (F82 and F83). It has an 84 mm bore and 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 7 849 620. BMW describes it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 85 mm and its thickness 1.20 mm. The repair size, 11 12 7 849 621, measures 1.50 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 MLS gaskets withstands 517 N/mm², 5 times more, and springs back at every combustion.

This page offers the part number from Cometic, a racing manufacturer, in MLS multi-layer steel construction: an 85 mm gasket bore and a thickness of 1.12 mm, close to the original 1.20 mm. 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: it is the space the gasket creates above each of the cylinders. That space is added to the combustion chamber, and a larger volume translates into a lower compression ratio. To compare these values, the gasket bore must be the same.

Preparation and fitting

Before fitting, check the flatness of the cylinder head and of the block with a straight edge and have them skimmed if necessary. This gasket requires no sealant: Cometic supplies it already coated.

Technical guide

The construction offered

Cometic MLS Several stainless steel layers, each one stamped, with the whole pack riveted. On the outer layers, active beads encircle every cylinder and every fluid passage. A moderate clamping load is all it takes to seal, and the bore distorts only slightly. It is the most versatile construction, suited from a tuned naturally aspirated engine to light boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS comes in 85 mm.

Thickness. The original gasket is 1.20 mm thick. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

When the chosen thickness is lower than the original one, measure with a dial gauge how far your pistons stand proud of the block: that protrusion governs the clearance remaining between the piston and the cylinder head.

The brand

Set up in 1989 in Concord, Ohio, Cometic Gasket, Inc. manufactures engine gaskets and sealing solutions in the United States. The company specialises in the gasket produced to order, to the bore and thickness wanted. Each of its products is made on American soil, 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 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 7 849 620 in standard size, 11 12 7 849 621 in repair size.

Important: the N55B30, also fitted to the M2 (F87) before the M2 Competition, receives a different original cylinder head gasket, part numbers 11 12 7 599 212 and 11 12 7 599 213. It is not covered by this page.

Why switch to a reinforced gasket?

Beyond the point where the original gasket lets go, the reinforced one keeps its grip: under increased combustion pressure it holds its geometry instead of collapsing.

The slightest power gain, whether it comes from a turbo, a supercharger or an increased compression ratio, makes the head gasket the most exposed link of the top end. Choosing a reinforced gasket then becomes a safety precaution more than a comfort: it is what keeps away the failure and the cylinder head removal it forces.