Reinforced cylinder head gasket BMW M50B25 M52B25 M52B28

The BMW engines covered

The BMW M50 (2.5 litres) and M52 (2.5 and 2.8 litres) are the 24-valve inline six-cylinder petrol engines fitted to the 3 Series (E36), 5 Series (E34 and E39), 7 Series (E38) and the Z3. The M50B25, M52B25 and M52B28 share the same 84 mm bore and, until September 1998, the same cylinder head gasket.

This original gasket carries the part number 11 12 1 726 619. It is an asbestos-free metal and fibre composite, with an 85 mm bore and a thickness of 1.75 mm. BMW offers a single repair size, 11 12 1 726 622 in 2.05 mm, intended for skimmed cylinder heads. It is sized for standard combustion pressure and settles as soon as 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. The standard composite is crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of the MLS and MLX gaskets withstands 517 N/mm², 5 times more than the original, 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 three constructions: O-Ring with a stainless ring, MLS multi-layer steel and MLX with a chamber stopper. Gasket bores run from 84.5 to 87.5 mm, covering the standard block as well as the rebored block, and thicknesses from 0.69 to 3.61 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 three constructions offered

	Athena O-Ring / Cut Ring A steel sheet faced with asbestos-free aramid fibre, with a stainless steel compression ring around each cylinder. The ring concentrates clamping force on a reduced diameter, where sealing is decided, and a silicone bead protects the water and oil passages. It is the construction that takes the highest pressures, and the only one that accepts a cylinder head already prepared with a compression wire.
	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.
	Cometic MLX The same base as the MLS, with a folded centre layer forming a stopper around the combustion chamber and a high-temperature coating. Local clamping increases without adding bore distortion. It is the most resistant of the all-steel gaskets, to be chosen when boost goes beyond what an MLS can hold, in 85 mm only.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 84.5, 85.5, 86 and 87 mm, the Cometic MLS in 85, 86, 86.5, 87 and 87.5 mm, the Cometic MLX in 85 mm only.

Thickness. The original gasket is 1.75 mm thick. 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 the original, 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 1 726 619 in standard size, 11 12 1 726 622 in repair size.

Important: the M52TU, fitted from September 1998 on the E46, E39 and Z3, receives a different original cylinder head gasket, part numbers 11 12 7 501 304 and 11 12 7 501 305. Those engines are not covered by the part numbers on this page, nor is the M50B20, which also receives a gasket of its own.

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.