Reinforced cylinder head gasket BMW B48B20

The BMW engines covered

The BMW B48B20 petrol engine, a 2.0 litre unit, powers the BMW models in the compatibility table below.

This original gasket carries the part number 11 12 8 654 272. Its bore is 83.3 mm and its thickness 0.70 mm; BMW lists it as an asbestos-free cylinder head gasket without specifying its construction, and offers a repair size, 11 12 8 654 273, of 1.00 mm. It is designed for standard combustion pressure: when that pressure rises, the clamping force drops, the cylinder head lifts slightly and coolant enters a cylinder.

Reinforced cylinder head gaskets

The head gasket does not seal on its own: sealing depends on the fasteners, which clamp the head against the block. Beyond 100 N/mm² of allowable stress a composite gasket is crushed for good; the 301 stainless steel of MLX gaskets withstands 517 N/mm², 5 times more, and springs back at every combustion.

This page offers the part numbers from Cometic, the American maker of competition gaskets, in a single construction: the MLX with a combustion chamber stopper. The gasket bore, 83.5 mm, is 0.2 mm larger than the original gasket, and two thicknesses are offered, 0.81 mm and 1.02 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

A gasket's volume is the free space it leaves over each cylinder. That space adds to the combustion chamber and lowers the compression ratio, the more so the larger it is. Only compare values between gaskets of the same bore.

Preparation and fitting

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

Technical guide

The construction offered

Cometic MLX A multi-layer stainless steel gasket whose folded centre layer forms a stopper around the combustion chamber, with a high-temperature coating. Clamping rises locally without adding to bore distortion. It is the most resistant of the all-steel gaskets, to be kept for severe boost; it is offered here in 83.5 mm only.

Choosing bore and thickness

Gasket bore. The Cometic MLX is only offered here in 83.5 mm (C14185).

Thickness. The original gasket is 0.70 mm thick, and 1.00 mm in the repair size. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

As both gaskets are thicker than the original (0.70 mm), they do not bring the piston closer to the cylinder head. If the head has been resurfaced, still use a dial gauge to measure how far the pistons protrude above the block: that figure gives the clearance that remains between piston and cylinder head.

The brand

Cometic Gasket, Inc. was founded in 1989 in Concord, Ohio, and designs gaskets and sealing solutions for engines. It built its name on the made-to-order gasket, in the bore and thickness required. Production remains entirely in the United States, using American raw 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 83.3 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 8 654 272 in the standard size, 11 12 8 654 273 in the repair size.

Important: the part numbers on this page only apply to the B48B20 engine: check your vehicle's engine code before placing an order.

Why switch to a reinforced gasket?

When the original gasket reaches its limits, the reinforced gasket steps in: under higher combustion pressure it holds its shape instead of sagging.

On a turbocharged or supercharged engine, or one with a raised compression ratio, the head gasket is the first part to fail in the top end. Choosing a reinforced gasket is as much about safety as comfort: it avoids the breakdown and the head removal that follows.