Reinforced cylinder head gasket Chevrolet Cavalier and Pontiac Grand Am LD9

The Chevrolet engines covered

The LD9 Quad 4 petrol engine powers several Buick, Chevrolet, Oldsmobile and Pontiac models, detailed below.

No manufacturer data is published on the original gasket: neither its part number, nor its bore, nor its construction, nor its thickness. Sized for the standard combustion pressure, it is no longer enough once that pressure rises: the clamping load fades, the head lifts slightly and coolant reaches a cylinder.

Reinforced cylinder head gaskets

Sealing does not depend on the gasket alone: the fasteners that clamp the head to the block contribute just as much. The stainless steel of an MLS gasket holds its load under high pressure, where a conventional gasket gives way.

This page presents the two part numbers of the American maker Cometic, a specialist in racing gaskets, in a single construction: MLS, a three-layer stainless steel multilayer. Their bore is 91.31 mm and their thickness 1.30 mm. The first is intended for engines with exhaust gas recirculation (EGR), the second for those without.

Technical specifications

Gasket volume is the space it leaves open above each cylinder; it enlarges the combustion chamber and lowers the compression ratio. Comparison only makes sense between gaskets of equal bore.

Preparation and fitting

Before fitting, check with a straightedge that the head and block are flat, and have them resurfaced if needed. Cometic inspects every MLS gasket before shipping; apply no sealing paste.

Technical guide

The construction offered

Cometic MLS Stamped, riveted stainless steel sheets stacked in three layers. The outer sheets carry active beads around each cylinder and each fluid passage. It seals with a moderate clamping load and distorts the bore little. Suited from the prepared naturally aspirated engine to moderate boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS gaskets exist in 91.31 mm (C5298-051 and C5866-051), with no other bore.

Thickness. For the original gasket, the manufacturer states no thickness. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

With no known original thickness, use a dial gauge to read how far the pistons protrude above the block face, above all if the removed gasket is thicker than 1.30 mm: this figure gives the clearance left between piston and head.

The brand

Cometic Gasket, Inc., an American company founded in 1989 in Concord, Ohio, designs gaskets and sealing solutions for engines. It became known for made-to-measure gaskets, cut to the bore and thickness wanted, made in the United States from 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 91.31 mm bore of the gaskets offered 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: not published by the manufacturer.

Important: C5298-051 suits LD9 engines with exhaust gas recirculation (EGR), C5866-051 those without: check whether your engine has EGR before ordering.

Why switch to a reinforced gasket?

When the original gasket is no longer enough, the reinforced gasket takes over: under higher combustion pressure it keeps its shape instead of crushing.

When the engine is pushed, with a turbo, a supercharger or a higher compression ratio, the head gasket is often the first part to fail. A reinforced gasket prevents the leak, and the head removal that would follow costs far more than the part.