Reinforced head gasket BMW 318is 318i 318ti Z3 1.8 1.9 16V M42B18 M44B19

The BMW engines covered

The BMW M42B18 is the 1.8-litre 16-valve DOHC inline four-cylinder petrol engine of the 318is (E30 and E36) and 318i (E36); the 1.9-litre M44B19 succeeded it in the E36 and the Z3. This page covers both engines: they take the same reinforced cylinder head gaskets.

On the M44B19, the original gasket carries part number 11 12 1 433 950 or 11 12 1 433 951, with an 86 mm bore. On the M42B18, it carries part number 11 12 1 721 546, or 11 12 1 721 547, 11 12 1 721 550 and 11 12 1 721 551 depending on the version. BMW publishes neither its construction nor its thickness; its bore is 85 mm. It is sized for standard combustion pressure: when that pressure rises, clamping load drops, the cylinder head lifts, and coolant passes into 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 brings together the part numbers from two racing manufacturers, Athena and Cometic, in two constructions: Athena Cut Ring with stainless rings and Cometic MLS multi-layer steel. Gasket bores run from 85 to 87.5 mm, covering the standard block as well as the rebored block, 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, one gasket per engine.

Technical specifications

Gasket volume: the space the gasket leaves above each cylinder. It enlarges the combustion chamber, and the larger it is, the lower the compression ratio. Values compare with one another at identical gasket bore.

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. None of these gaskets requires sealant: Athena and Cometic both supply them already coated.

Technical guide

The two constructions offered

Athena Cut Ring A gasket in reinforced, asbestos-free MO115 material (steel core, aramid fibres, NBR binder), supplied with separate AISI 304 stainless steel rings around each cylinder. Athena also intends it for turbocharged or nitrous engines above 2 bar, with no machining of the cylinder head or block. A silicone bead reinforces the sealing of the water and oil passages. On fitting, the ring embossments face the cylinder head. Athena rates the MO115 material at 100 N/mm² at most: the stainless rings are what hold combustion.

Cometic MLS Several stamped and riveted stainless steel layers. The outer layers carry active beads around each cylinder and each fluid passage. Sealing is achieved with a moderate clamping load and limited bore distortion. This is the versatile choice, from a prepared naturally aspirated engine to light forced induction.

Choosing bore and thickness

Gasket bore. The Athena Cut Ring comes in 86.5 mm, the Cometic MLS in 85, 86, 86.5, 87 and 87.5 mm.

Thickness. BMW does not publish the thickness of the original gasket. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

With a thickness finer than that of the removed gasket, measure piston protrusion above the block with a dial gauge: it is what determines the remaining clearance between piston and cylinder head.

The two brands

	Athena S.p.A. is an Italian manufacturer of engine gaskets and components, with its head office and main plant in Alonte, in the Veneto region. The group also produces in Spain, the United States, Brazil and India. Its multi-layer gaskets are fitted as original equipment by car manufacturers, and its racing range is designed and made entirely in Italy.
	Cometic Gasket, Inc. is an American manufacturer of engine gaskets and sealing solutions, founded in 1989 and based in Concord, Ohio. Its speciality is the made-to-order gasket, in the bore and thickness required. All its products are made 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 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 1 721 546, 11 12 1 721 547, 11 12 1 721 550, 11 12 1 721 551 (M42B18) ; 11 12 1 433 950, 11 12 1 433 951 (M44B19).

Important: the 8-valve M40B16 and M43B16 take another cylinder head gasket (11 12 7 504 586): they are not covered by this page.

Why switch to a reinforced gasket?

A reinforced gasket holds where the standard one reaches its limit: at higher combustion pressure it keeps its shape instead of being crushed.

As soon as power goes up, turbo, supercharger or raised compression ratio, the head gasket becomes the weak link of the top end. Switching to a reinforced gasket is then not a comfort but a safeguard: it is what prevents the failure and the head removal that comes with it.