Reinforced cylinder head gasket Athena and Cometic 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. With every combustion event, pressure tries to separate the two parts, and the gasket must stay tight despite that varying load. A composite eventually settles; a reinforced gasket works in steel, whose stamped beads or compression ring recover their shape on every cycle.

This page brings together 36 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 in 24 values, enough to adjust the compression ratio without touching the pistons or the cylinder head. Bores above 85 mm are intended for rebored or resleeved M50 cast iron blocks. Each part number is sold individually, one gasket per engine.

Technical specifications

Added 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

Original cylinder head bolts are tightened to torque and then by angle, in two 90° passes. They stretch permanently: BMW requires them to be replaced, cleaned and oiled every time the cylinder head is removed. ARP studs are tightened by torque alone, in three passes, with the assembly lubricant supplied in the kit, and are reusable.

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. Cometic does, however, rule out fitting an MLS or an MLX on a cylinder head prepared with a compression wire.

A reinforced gasket does not call for a higher torque than the manufacturer's, but for a clamping load that is even and stable over time. ARP cylinder head studs hold it better than bolts that stretch permanently, and they are reusable. The ARP kit in our catalogue covers the M50B25 and the M50B25TU; on the M52, fitting is done with a new set of original bolts.

Technical guide

Three elements set the 36 part numbers apart: construction, gasket bore and thickness.

The three constructions offered

	Athena O-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 only construction 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 construction most resistant to high boost pressures, 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 designs and manufactures its gaskets in Italy. The O-Ring range belongs to its racing line: a steel sheet covered with asbestos-free aramid fibre receives an AISI 304 stainless steel ring around each cylinder, and a silicone bead is applied to the water and oil passages. This is the construction the brand uses for heavily boosted engines.
	Founded in 1989 by Bob Gorman, Cometic designs and manufactures its gaskets in the United States, in Concord, Ohio, and validates its constructions on its own engine dynamometer. The gaskets are formed of several stamped and riveted stainless steel layers, drawn to the requested bore and thickness rather than to the original size.

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 is this the most economical solution?

A cylinder head gasket that fails never costs the price of a gasket. It costs removing the cylinder head, a new set of bolts, a skim, coolant and labour, with the risk of a warped head. On a prepared engine, the price difference against the original gasket is recovered the first time a strip-down is avoided.

The same reasoning applies to thickness. The gasket is the only component that allows the compression ratio to be adjusted without touching the pistons or the cylinder head: changing thickness costs the price of a gasket, where going through the pistons or a skim commits a budget of an altogether different order.