Reinforced head gasket Porsche 911 Turbo GT3 GT2 RS

The Porsche engines covered

The Porsche engines on this page, of 3.8 and 4.0 litres capacity, power the 911 in its 997, 991 and 992 generations, as Turbo, Turbo S, GT3, GT3 RS and GT2 RS, as shown in the table below. The gaskets are sold individually and by cylinder bank: one is needed on the left and one on the right for each engine.

The original gasket carries two series of part numbers: 0PB 103 148 B, 9A1 104 147 03, 9A1 104 149 00 and 9A1 104 149 05 on one hand, 0PB 103 149 B, 9A1 104 146 03 and 9A1 104 148 00 on the other. Porsche publishes neither its bore, nor its thickness, nor its construction. It is sized for standard combustion pressure: when that rises, the clamping load drops, the head moves away from the block and coolant reaches a cylinder.

Reinforced cylinder head gaskets

The gasket is not solely responsible for the seal: the fasteners that clamp the head onto the block count just as much. A composite gasket settles for good above 100 N/mm², whereas the 301 stainless steel of the MLX withstands 517 N/mm², 5 times more, and recovers its shape on every combustion. With the O-Ring, the stainless ring closes the combustion chamber on its own, and the gasket body seals the water and oil circuits, with no machining of the block.

This page brings together the part numbers of two racing manufacturers, Athena and Cometic, in two constructions: the stainless-ring O-Ring from Athena and the chamber-stop MLX from Cometic. Gasket bores run from 103 to 105 mm and thicknesses from 0.813 to 1.321 mm, enough to tune the compression ratio without touching the pistons or the 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 and for one cylinder bank only: one is needed on the right and one on the left.

Technical specifications

The volume of a gasket is the room it frees above each cylinder. This space is added to the combustion chamber, and the compression ratio falls by the same amount. Compare these volumes only between gaskets of the same bore.

Preparation and fitting

When fitting, first check with a straight edge that the cylinder head and the block are flat, and have them skimmed if necessary. No sealant is needed: Athena and Cometic supply their gaskets pre-coated.

Technical guide

The two constructions offered

	O-Ring / Cut Ring Athena A steel core backed with asbestos-free aramid fibre, onto which a stainless steel compression ring is crimped around each cylinder. The ring concentrates the clamping force on a narrow band, exactly where sealing is decided, while a silicone bead protects the water and oil passages. It is the construction in the range that withstands the highest combustion pressures.
	MLX Cometic Stamped, riveted stainless steel plates, including a central plate folded into a stopper around the combustion chamber, under a high-temperature coating. The clamping force is localised where it is needed, without increasing bore distortion. It is the most resistant all-steel gasket, for high boost levels; it is offered here in 105 mm, in three thicknesses from 0.813 to 1.321 mm.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 103 and 105 mm, the Cometic MLX in 105 mm, each in a left and a right version.

Thickness. Porsche communicates no thickness for the original gasket. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

The manufacturer gives no original thickness, so use a dial gauge to read how far the pistons stand proud of the block's gasket face: this figure sets the distance remaining between piston and cylinder head, particularly with the thinnest gasket.

The two brands

	An Italian company making gaskets and parts for engines, Athena S.p.A. has its head office and main plant in Alonte, in the Veneto, with additional sites in Spain, the United States, Brazil and India. Its multi-layer gaskets are fitted as original equipment by car manufacturers, and the entire competition range is born and built in Italy.
	Founded in 1989 in Concord, Ohio, Cometic Gasket, Inc. is an American company that makes gaskets and sealing solutions for engines. It has specialised in the made-to-measure gasket, in the bore and thickness requested, produced entirely 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 bore of the gaskets offered (103 to 105 mm) 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: 0PB 103 148 B, 9A1 104 147 03, 9A1 104 149 00, 9A1 104 149 05 ; 0PB 103 149 B, 9A1 104 146 03, 9A1 104 148 00.

Important: the gaskets on this page are listed for the Porsche 911 engines (997, 991 and 992) in the table. For any other vehicle, note the engine code before ordering.

Why switch to a reinforced gasket?

Once the original gasket has reached its limit, the reinforced gasket takes over the load: at a higher combustion pressure it keeps its shape instead of settling.

When power climbs, through an added turbo, a supercharger or a higher compression ratio, the head gasket becomes the most stressed part of the top end. Choosing a reinforced gasket means avoiding the leak, and then the removal of the cylinder head, a job out of all proportion to the price of the part.