Reinforced head gasket Porsche 911 996 997 Turbo GT2 GT3 M96 M97

The Porsche engines covered

The Porsche M96.70, M96.76, M96.79, M97.70, M97.70S and M97.76 petrol engines, 3.6-litre flat-sixes, are fitted to the Porsche 911 (996) and 911 (997) models, Turbo, GT2 and GT3 versions included, listed below.

The original gasket part number for these engines is 997 104 170 71. Porsche publishes neither the bore, nor the construction, nor the thickness. Each engine takes two, one per cylinder bank. It is designed for standard combustion pressure: as that pressure rises, the clamping load relaxes, the cylinder head lifts and coolant seeps into a cylinder.

Reinforced cylinder head gaskets

Sealing does not depend on the gasket alone: the fasteners clamping the head to the block count as much as it does. Above 100 N/mm², a composite gasket compresses for good; the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more, and springs back at every combustion.

This page brings together the part numbers of Cometic, an American maker of racing gaskets, for a single construction: the multi-layer steel MLS. Bores run from 102 to 106.5 mm and thicknesses from 0.69 to 3.56 mm. 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: allow for two gaskets per engine, one per cylinder bank.

Technical specifications

A gasket's volume is the space it leaves open above each cylinder. It adds to the combustion chamber, and the larger it is, the more the compression ratio drops. Only compare this figure between gaskets of the same bore.

Preparation and fitting

Before reassembly, check with a straight edge the flatness of the gasket faces of each cylinder head and of the block, and have them resurfaced if needed. Use no sealing paste: Cometic gaskets arrive already coated.

Technical guide

The construction offered

MLS Cometic Stainless steel layers, embossed and assembled with rivets. The outer layers have active beads around each cylinder and each fluid passage. A moderate tightening torque is enough to seal and the bore distorts very little. This versatile design runs from the tuned naturally aspirated engine to moderate boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS comes in 102 mm (C4492), 104 mm (C14077 and H2457), 104.5 mm (C14076) and 106.5 mm (C14049).

Thickness. The thickness of the original gasket is not published by Porsche. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

With a thickness below the original, use a dial gauge to read how far the pistons stand above the block deck: the reading gives the clearance left between piston and head.

The brand

Cometic Gasket, Inc. is an American maker of engine gaskets and sealing solutions, founded in 1989 in Concord, Ohio. Its speciality is the made-to-measure gasket, cut to the bore and thickness wanted. All production stays in the United States, with 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 original M96 and M97 gasket, which Porsche does not publish 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: 997 104 170 71.

Important: the part numbers on this page fit only the M96.70, M96.76, M96.79, M97.70, M97.70S and M97.76 engines: check your vehicle's engine code before placing an order.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, the reinforced gasket steps in: under higher combustion pressure it keeps its shape and does not crush.

Fit a turbo or a supercharger, or raise the compression ratio, and the head gasket becomes the most stressed part of the top end. A reinforced gasket prevents the leak, and the head removal that would follow. On a flat-six, each cylinder bank has its own.