Reinforced cylinder head gasket BMW M20B25 M20B27

The BMW engines covered

The BMW M20B25 (2.5 litres) and M20B27 (2.7 litres, the "eta" engine) are inline six-cylinder petrol engines fitted to the 3 Series (E30), the 5 Series (E28 and E34) and the models detailed further down. Their capacities differ, but both engines take the same original cylinder head gasket, on which every part number on this page is based.

This original gasket carries the part number 11 12 1 722 734, since replaced by the 11 12 1 722 737 in standard size. It is a soft-metal composite gasket with an 85 mm bore and a thickness of 1.75 mm. BMW offers a single repair size, 11 12 1 722 738 at 2.05 mm, intended for skimmed cylinder heads and now discontinued. 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 head gasket's seal does not rest on the gasket alone: it comes from the fasteners, which clamp the cylinder head onto the block. The original composite is permanently crushed beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of MLS gaskets takes 517 N/mm², 5 times more than the original, and returns to its shape after every combustion. The O-Ring works on another principle: its stainless ring alone seals the chamber, while the gasket body handles water and oil, with no machining of the block.

This page gathers the part numbers of two racing manufacturers, Athena and Cometic, in two constructions: the O-Ring with a stainless steel ring and the multi-layer steel MLS. Gasket bores run from 85 to 86.5 mm, enough to cover the standard block as well as a 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, one gasket per engine.

Technical specifications

Gasket volume: the free space left by the gasket above each cylinder. It adds to the combustion chamber, and a larger volume lowers the compression ratio. Only compare these figures between gaskets of the same bore.

Preparation and fitting

Before fitting the gasket, check with a straight edge that the cylinder head and block are flat, and have them skimmed if necessary. No sealant is needed: Athena and Cometic supply these gaskets already coated.

Technical guide

The two constructions offered

Athena O-Ring / Cut Ring A steel sheet dressed in asbestos-free aramid fibre, fitted around each cylinder with a stainless steel compression ring. The ring concentrates the clamping load on a small diameter, right where the sealing happens, and a silicone seal protects the water and oil passages. It is the construction that holds 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 joined with rivets. The outer layers carry active embossments around each cylinder and each fluid channel. A moderate tightening torque is enough to seal, and the bore distorts very little. It is the all-round answer, from a tuned naturally aspirated engine to moderate forced induction.

Choosing bore and thickness

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

Thickness. The original gasket measures 1.75 mm, and 2.05 mm in repair size (+0.30 mm). A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If the chosen thickness is below the original, use a dial gauge to read how far the pistons stand proud of the block: that figure gives the clearance remaining between piston and cylinder head.

The two brands

	Athena S.p.A., an Italian maker of engine gaskets and components, is based in Alonte, in the Veneto region, where its main factory is also located. It has production sites in Spain, the United States, Brazil and India as well. Car manufacturers use its multi-layer gaskets as original equipment, and its whole racing range is designed and built in Italy.
	Cometic Gasket, Inc. is an American maker of engine gaskets and sealing solutions, set up in 1989 in Concord, Ohio. It stands out for the made-to-measure gasket, produced to the bore and thickness you need. All of its production takes place 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 1 722 734, replaced by the 11 12 1 722 737 in standard size; 11 12 1 722 738 in repair size.

Important: the M20B20 and M20B23 take a different original cylinder head gasket (part number 11 12 1 722 728, 81 mm bore) and are not covered by the part numbers on this page: they have their own page. Check your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches the end of its capacity, the reinforced one takes over: even under higher combustion pressure it keeps its shape and is not crushed.

With power on the rise, whether through a turbo, a supercharger or a raised compression ratio, the head gasket is the first link to give way in the top end. Choosing a reinforced gasket therefore means putting reliability first: it spares you the failure and the cylinder head removal that comes with it.