Reinforced head gasket Toyota NASCAR V8

The Toyota engines covered

The Toyota NASCAR V8 racing engine takes two cylinder head gaskets, one per cylinder bank. It powers no production vehicle: it is intended for competition cars.

The manufacturer publishes nothing on the original gasket: no part number, no bore, no construction and no thickness. Designed for the standard combustion pressure, it falls short as soon as that pressure increases: the clamping load relaxes, the head opens slightly and coolant enters a cylinder.

Reinforced cylinder head gaskets

Sealing does not rest on the gasket alone: the bolts or studs that hold the head down on the block play an equal part. The stainless steel of an MLS gasket keeps its load at high pressure, where a conventional gasket settles.

This page offers two part numbers from the American maker Cometic, a specialist in racing gaskets, in one construction only: MLS, a three-layer stainless steel multilayer. Their bore is 106.68 mm and their thickness 1.02 mm. The V8 takes one gasket per cylinder bank: order both part numbers, C4291-040 for the left bank and C4290-040 for the right bank.

Technical specifications

The gasket volume is the space left open above each cylinder: it adds to the combustion chamber and lowers the compression ratio. Only gaskets of the same bore can therefore be compared.

Preparation and fitting

Use a straightedge to check the flatness of the head and the block before fitting, and have any distorted surface machined. Every Cometic MLS gasket is inspected before it leaves the factory; coat it with no sealing paste.

Technical guide

The construction offered

Cometic MLS Three stainless steel sheets, stamped and riveted together in a stack. The outer sheets show active beads around each cylinder and each fluid passage. It seals with a moderate clamping load and alters the bore very little. It is used from the prepared naturally aspirated engine up to moderate boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS gaskets exist in 106.68 mm (C4291-040 and C4290-040), with no other bore.

Thickness. The manufacturer gives no thickness for the original gasket: only the dimension of the removed gasket serves as a reference. 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 1.02 mm: this figure gives the clearance left between piston and head.

The brand

Cometic Gasket, Inc. is a company from the United States, set up in 1989 in Concord, Ohio, that designs gaskets and sealing parts for engines. It earned its reputation with made-to-measure gaskets, cut to the required bore and thickness and produced on site 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 106.68 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: The V8 takes one gasket per cylinder bank: C4291-040 for the left bank, C4290-040 for the right bank. Order both part numbers for a complete engine.

Why switch to a reinforced gasket?

When the original gasket no longer holds, the reinforced gasket steps in: at higher combustion pressure it keeps its shape instead of flattening.

On an engine under strain, with a turbo, a supercharger or a raised compression ratio, the head gasket often gives up before any other part. The reinforced gasket prevents the leak, and the head removal that a leak would force costs far more than the gasket.