Reinforced cylinder head gasket BMW M52B25TU M52B28TU M54B25 M54B30

The BMW engines covered

The BMW M52TU (2.5 and 2.8 litres) and M54 (2.5 and 3.0 litres) are the 24-valve inline six-cylinder petrol engines fitted to the 3 Series (E46), 5 Series (E39 and E60), 7 Series (E38 and E65), the X3 (E83) and X5 (E53), the Z3 and the Z4 (E85). The M54B25 and M54B30 have an 84 mm bore, and all four engines receive the same original cylinder head gasket, fitted from September 1998.

This original gasket carries the part number 11 12 7 501 304. BMW describes it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 85 mm and its thickness 0.70 mm. The repair size, 11 12 7 501 305, measures 1.00 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 head gasket does not seal by itself: it keeps the seal because the fasteners press the cylinder head onto the block. Beyond 100 N/mm² of admissible stress a composite gasket is permanently crushed, while the 301 stainless steel used in MLS gaskets withstands 517 N/mm², 5 times more, and recovers its shape with each combustion. The O-Ring shares out the roles differently: its stainless ring alone seals the combustion, the gasket body looks after the water and oil circuits, and the block needs no machining.

This page brings together the part numbers from two racing manufacturers, Athena and Cometic, in two constructions: O-Ring with a stainless ring and MLS multi-layer steel. Gasket bores run from 84.5 to 87.6 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 free space the gasket leaves above each cylinder. It adds to the combustion chamber, so the larger it is, the lower the compression ratio falls. Only compare these values at the same gasket bore.

Preparation and fitting

Before assembly, check with a straight edge that the cylinder head and the block are flat, and have them skimmed if needed. No sealant is required with any of these gaskets: Athena and Cometic deliver them all pre-coated.

Technical guide

The two constructions offered

Athena O-Ring / Cut Ring A steel sheet coated with asbestos-free aramid fibre, fitted with a stainless steel compression ring around every cylinder. The ring focuses the clamping force on a small diameter, exactly where the seal is made, while a silicone bead protects the water and oil passages. It is the construction that copes with the highest pressures, and the only one suited to a cylinder head already prepared with a compression wire.

Cometic MLS A stack of stainless steel layers, stamped and then riveted together. Around every cylinder and every fluid passage, the outer layers feature active beads. A moderate clamping load is enough to seal, with bore distortion kept low. It is the versatile option, from a tuned naturally aspirated engine up to mild forced induction.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 84.5, 86 and 87.6 mm, the Cometic MLS in 85 mm.

Thickness. The original gasket is 0.70 mm thick. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If you pick a thickness below the original, use a dial gauge to measure how far the pistons protrude above the block: that figure sets the clearance left between piston and cylinder head.

The two brands

	An Italian maker of engine gaskets and components, Athena S.p.A. has its head office and main factory in Alonte, in the Veneto region. The group also runs production sites in Spain, the United States, Brazil and India. Car manufacturers fit its multi-layer gaskets as original equipment, and its racing range is entirely designed and built in Italy.
	Founded in 1989 and located in Concord, Ohio, Cometic Gasket, Inc. is an American maker of engine gaskets and sealing solutions. Its core business is the gasket built to order, to the bore and thickness requested. Its entire output is manufactured 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 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 7 501 304 in standard size, 11 12 7 501 305 in repair size.

Important: the M52B25 and M52B28 fitted before September 1998, as well as the M50B25, receive a different original cylinder head gasket, part numbers 11 12 1 726 619 and 11 12 1 726 622. Those engines are not covered by the part numbers on this page; the 523i, 528i (E39), 728i (E38) and Z3 built before September 1998 are among them.

Why switch to a reinforced gasket?

Where the original gasket reaches its limit, the reinforced one keeps holding: as combustion pressure climbs, it retains its shape rather than being crushed.

Turbo, supercharger or a higher compression ratio: once power rises, the head gasket turns into the weakest link of the top end. Moving to a reinforced gasket is then a matter of safety rather than comfort: it avoids the failure, and the cylinder head removal that follows it.