Reinforced head gasket Jaguar XJ XJ-S AJ6 3.6

The Jaguar engines covered

The AJ6 3.6-litre petrol engine, an inline six, powers the Jaguar XJ (XJ40, XJ81), XJ Coupé and XJSC Convertible (X27) listed below.

The original gasket of these engines carries the part number NBC2520AB, which serves the 3.6-litre of the XJ-S and of the XJ40 from engine 109270. Jaguar publishes neither its bore, nor its construction, nor its thickness. It is sized for the original combustion pressure: if it rises, the clamping load eases, the head lifts and coolant passes into a cylinder.

Reinforced cylinder head gaskets

Sealing does not depend on the gasket alone: the fasteners that clamp the head onto the block matter just as much. Copper of 99.9% purity resists corrosion, deforms without breaking and conducts heat well, with no brittleness under temperature.

This page describes the part number from the American maker Cometic, a specialist in racing gaskets, in a single construction: copper. Its bore is 92.5 mm and its thickness 1.60 mm. By convention, the gasket bore is at least 0.5 mm larger than that of the cylinders. It is sold individually: one gasket per engine.

Technical specifications

Gasket volume is the space the gasket leaves open above each cylinder. It adds to the combustion chamber, and the larger it is, the lower the compression ratio. Only gaskets of the same bore can be compared with one another.

Preparation and fitting

Before fitting, check with a straight edge that the head and block faces are flat, and have them resurfaced if necessary. Cometic checks the flatness of each of its copper gaskets.

Technical guide

The construction offered

Choosing bore and thickness

Gasket bore. The Cometic copper gasket is offered in 92.5 mm (H2954063C), with no other bore.

Thickness. Jaguar gives 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.

With no published original thickness, use a dial gauge to measure how far the pistons stand proud of the block deck, especially if the removed gasket is thinner than 1.60 mm: it shows the clearance left between piston and head.

The brand

Cometic Gasket, Inc., founded in 1989 in Concord, Ohio, is an American maker of engine gaskets and sealing solutions. Its reputation rests on the made-to-measure gasket, cut to the bore and thickness wanted. All production stays in the United States, using 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 92.5 mm bore of the gasket 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: NBC2520AB.

Important: the part number on this page suits the AJ6 engine of 3.6 litres: check the displacement and engine code of your vehicle before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, this copper gasket takes over: pure copper is ductile and does not turn brittle when hot, so the seal between head and block is kept.

Turbo, supercharger or a raised compression ratio: the head gasket takes the strain first, the weak point of the top end. A reinforced gasket prevents the leak and the head removal that goes with it, a job far dearer than the gasket itself.