Reinforced head gasket Porsche 924 2.0 Turbo XJ XK M31.01 M31.03

The Porsche engines covered

The XJ, XK, M31.01 and M31.03 petrol engines are 2.0 litre in-line four-cylinders with two valves per cylinder. They power the Porsche 924 2.0 and 924 2.0 Turbo listed in the compatibility table below.

The original gasket carries part number 931.104.337.03. The manufacturer publishes neither its bore, nor its thickness, nor its construction. An engine takes a single cylinder head gasket, sized for the standard combustion pressure; when that pressure climbs, the clamping force eases, the head lifts slightly and coolant seeps into a cylinder.

Reinforced cylinder head gaskets

Sealing also depends on the fasteners that clamp the head to the block, as much as on the gasket. Above 100 N/mm², a composite gasket is crushed permanently; the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more, and recovers its shape at every combustion. Also inspect the head bolts and change any that are elongated or damaged.

This page brings together 19 Cometic part numbers, from the American maker of competition gaskets, for a single construction: MLS, multi-layer steel. A single bore is offered, 88 mm, in 19 thicknesses from 0.69 to 3.56 mm. An engine takes a single gasket: order one part number. The thicknesses offered let you 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 above each cylinder; it adds to the combustion chamber and lowers the compression ratio. Compare only gaskets of the same bore: 88 mm here.

Preparation and fitting

Before reassembly, check the flatness of the head and block surfaces with a straightedge and have them machined if needed. Cometic gaskets come with their coating: do not add sealant, and clean both surfaces thoroughly before fitting.

Technical guide

The construction offered

Cometic MLS Several embossed stainless steel sheets, assembled with rivets. Each outer sheet has active ridges that follow the perimeter of every cylinder and every coolant passage. A moderate tightening torque ensures sealing, and the bore distorts little. This versatile gasket suits everything from the tuned naturally aspirated engine to moderate boost. All the part numbers on this page are offered in this construction.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in a single bore, 88 mm (C4542).

Thickness. No thickness is published for the original gasket 931.104.337.03. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If you are considering a gasket thinner than the original, use a dial gauge to measure how far each piston protrudes above the block deck: that protrusion determines the clearance left between piston and head. Record it on all four cylinders.

The brand

Cometic Gasket, Inc. was established in 1989 and is headquartered in Concord, Ohio. This American company designs and produces gaskets and sealing parts for engines. It made its name with made-to-measure gaskets, cut to the required bore and thickness, all manufactured 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 88 mm bore of these gaskets 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: 931.104.337.03.

Important: the part numbers on this page suit only the XJ, XK, M31.01 and M31.03 engines: check your Porsche's engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches the end of its capacity, the reinforced gasket holds on: under higher combustion pressure it keeps its shape instead of collapsing.

With a turbo added, a raised compression ratio or higher revs from livelier camshafts, the head gasket is the first weak point of the tuned engine. A reinforced gasket prevents the leak, and the head removal that follows costs far more than the gasket.