Reinforced cylinder head gasket BMW M30B34 and M30B35

The BMW engines covered

The BMW M30B34 (3.4 litres) and M30B35 (3.5 litres) are naturally aspirated twelve-valve inline-six petrol engines. They are fitted to the BMW 535i, 635 CSi, 735i and 745i, from the E24 coupe to the E12, E23, E28, E32 and E34 saloons listed below. 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 1 730 223, since replaced by the 11 12 1 730 226 in standard size. It is a soft-metal composite gasket with a 93.5 mm bore and a thickness of 1.72 mm. BMW offers a repair size, 11 12 1 730 227 at 2.07 mm (+0.30 mm), which replaces the 11 12 1 730 224. 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

The head gasket does not provide the seal on its own: the seal comes chiefly from the fasteners, which clamp the cylinder head against the block. The original composite is crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more than the original, and regains its shape after every combustion. The O-Ring rests on another principle: its stainless ring is enough to seal 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 93 to 95 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 the gasket leaves above each cylinder. It adds to the combustion chamber, and a larger volume lowers the compression ratio. Only compare these values between gaskets of the same bore.

Preparation and fitting

Before fitting, check with a straight edge that the cylinder head and block are flat, and have them machined if needed. No sealing paste is required: Athena and Cometic supply these gaskets already coated.

Technical guide

The two constructions offered

Athena O-Ring / Cut Ring A steel sheet lined with asbestos-free aramid fibre, with a stainless steel compression ring around each cylinder. The ring concentrates the clamping force on a small diameter, exactly where the seal is made, and a silicone seal protects the water and oil passages. This construction withstands the highest pressures and is the only one suited to a cylinder head already prepared with a compression wire.

Cometic MLS A pile of stamped stainless steel sheets, assembled afterwards with rivets. The outer sheets carry active beads around each cylinder and each fluid passage. A moderate tightening torque is enough to seal, and the bore hardly distorts. It is the versatile solution, from the tuned naturally aspirated engine to moderate forced induction.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 93.1 mm, the Cometic MLS in 93, 94, 94.5 and 95 mm.

Thickness. The original gasket measures 1.72 mm, and 2.07 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 one, measure with a dial gauge how far the pistons protrude above the block: it shows the clearance remaining between piston and head.

The two brands

	Athena S.p.A. is an Italian maker of gaskets and engine components, whose head office and main plant are in Alonte, in the Veneto region. It also runs production sites in Spain, the United States, Brazil and India. Car makers use its multi-layer gaskets as original equipment, and its whole racing range is designed and built in Italy.
	Cometic Gasket, Inc., an American maker of engine gaskets and sealing solutions, was founded in 1989 in Concord, Ohio. Its speciality is the made-to-order gasket, produced to the bore and thickness requested. All of its production stays in the United States and uses 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 further than the 93.5 mm bore of the original gasket allows
• 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 1 730 223, replaced by the 11 12 1 730 226 in standard size; 11 12 1 730 227 in repair size.

Important: the M30B25, M30B28, M30B30 and M30B32 take other original cylinder head gaskets and are not covered by the part numbers on this page: they have their own pages. Check your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

Once the original gasket reaches its limits, the reinforced gasket carries on: even under higher combustion pressure it holds its shape instead of being crushed.

As power climbs, through a turbo, a supercharger or a raised compression ratio, the head gasket is the first part to give up in the top end. Opting for a reinforced gasket therefore means betting on reliability: you spare yourself the failure, and the cylinder head removal that follows.