Reinforced cylinder head gasket BMW S65B40

The BMW engines covered

The BMW S65B40 is the naturally aspirated V8 petrol engine of 4.0 litres fitted to the BMW M3, in saloon (E90), coupé (E92) and convertible (E93) form. Its bore is 92 mm with a 75.2 mm stroke, and all its versions receive the same original cylinder head gasket, one per cylinder bank.

This original gasket carries the part number 11 12 7 841 560. BMW lists it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 92.8 mm and its thickness 0.60 mm, in a single size. The engine takes two identical ones, one per cylinder bank. It is intended for standard combustion pressure: as that pressure rises, clamping load fades, the cylinder head lifts and coolant passes into a cylinder.

Reinforced cylinder head gaskets

A head gasket does not make the seal by itself: the seal is maintained because the fasteners press the head onto the block. A composite gasket ends up crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of MLS gaskets takes 517 N/mm², 5 times more, and regains its shape at every combustion.

This page gathers the part numbers of Cometic, an American maker of racing gaskets, in a single construction: the multi-layer steel MLS. Gasket bores run from 93 to 94 mm, which covers the standard block as well as the rebored one, and thicknesses from 0.69 to 3.56 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; the engine takes two gaskets, one per cylinder bank, so order the part number for each bank.

Technical specifications

Gasket volume: the space the gasket leaves free above each cylinder. It joins the combustion chamber, and the larger it is, the more the compression ratio drops. Compare these volumes only between gaskets of the same bore.

Preparation and fitting

Before fitting the gasket, run a straight edge over the head and the block to check their flatness, and have them ground if necessary. These gaskets go on dry, without sealant, since Cometic delivers them already coated.

Technical guide

The construction offered

Cometic MLS An assembly of stamped stainless steel layers, riveted to each other. The outer layers carry active beads around each cylinder and each fluid passage. A moderate clamping load is enough to seal, and the bore distorts little. It is the versatile solution, from the prepared naturally aspirated engine to moderate forced induction.

Choosing bore and thickness

Gasket bore. The Cometic MLS comes in 93 mm (C5111) and in 94 mm (C5112).

Thickness. The original gasket is 0.60 mm thick, in a single size. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

Since no thickness offered is below the original, the gasket does not bring the pistons closer to the head: measuring piston protrusion with a dial gauge does not depend on the gasket chosen.

The brand

Founded in 1989 in Concord, Ohio, Cometic Gasket, Inc. makes gaskets and sealing solutions for engines. It has specialised in the gasket produced to order, in the bore and thickness asked for. All production takes place in the United States, using 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 92.8 mm bore of the standard 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 7 841 560, in a single size (two identical gaskets per engine).

Important: this engine takes two cylinder head gaskets, one per cylinder bank: order the part number for each bank. The part numbers on this page fit only the S65B40 of the M3.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, the reinforced one keeps holding: at higher combustion pressure it retains its shape rather than being crushed.

As soon as power goes up, whether with a supercharger, a turbo or a raised compression ratio, the head gasket is the first part of the top end to give way. Opting for a reinforced gasket is therefore about safety rather than comfort: it spares you the failure and the head removal that follows.