Reinforced head gasket Jaguar XJ E-Type 4.2 XK

The Jaguar engines covered

Jaguar's 4.2 litre XK inline six powers the Jaguar XJ and E-Type in the table below. A single head covers all six cylinders: the gasket is sold singly, one per engine.

The manufacturer part number of the standard gasket is NXD2520AA (XJ6 4.2 litre); neither the bore, nor the thickness, nor the construction appears in any manufacturer documentation. It is sized for the original combustion pressure: beyond it, the clamping load fades, the head lifts and coolant passes into a cylinder.

Reinforced cylinder head gaskets

A gasket alone cannot seal: the fasteners that clamp the head to the block count as much as it does. Beyond 100 N/mm², a composite gasket settles for good; the 301 stainless steel of the MLS carries 517 N/mm², 5 times more, and springs back to shape with every combustion. The raised features of the outer layers pass on the clamping load around each cylinder and each fluid passage, without any machining of the block.

This page gathers the part numbers of a single racing manufacturer, Cometic, in multi-layer MLS construction, with four gasket bores, from 93 to 93.726 mm. Thicknesses run from 0.686 to 3.556 mm, enough to adjust the compression ratio without altering either the pistons or the 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

Gasket volume stands for the space left open above each cylinder. It adds to the combustion chamber and lowers the compression ratio by the same amount. Restrict the comparison to gaskets of the same bore.

Preparation and fitting

Ahead of assembly, run a straight edge over the head and the block to check their flatness, and have the surface that needs it resurfaced. Add no sealing paste: Cometic gaskets are supplied already coated.

Technical guide

The construction offered

Cometic MLS Pressed stainless steel plates, laid over one another and riveted. The outer layers show active beads around each cylinder and each liquid passage. The gasket seals with a moderate clamping load and barely distorts the bore. It is the versatile solution, from the tuned naturally aspirated engine to sensible boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS comes in 93 and 93.218 mm (C4128) and in 93.5 and 93.726 mm (C4127).

Thickness. The manufacturer supplies no thickness for the standard gasket: only the reading of the gasket you remove counts. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

If no original thickness is published, measure with a dial gauge how far the pistons protrude above the block deck: that gives the clearance remaining between piston and head, particularly with the thinnest gasket.

The brand

Cometic Gasket, Inc., a US company founded in 1989 in Concord, Ohio, develops engine gaskets and sealing solutions. It has made a speciality of the custom gasket, at the bore and thickness requested, built entirely 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 bore of the gaskets offered (93 to 93.726 mm) 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: NXD2520AA.

Important: the gaskets on this page are listed for the 4.2 litre XK engine in the survey below, at Jaguar. For any other vehicle, find the engine code first before placing an order.

Why switch to a reinforced gasket?

When the standard gasket falls short, the reinforced one takes over: at higher combustion pressure it holds its geometry rather than sagging.

Demand more of an engine, with more boost, a higher compression ratio or a thorough build, and the head gasket is the first part in the top end to give up. A reinforced gasket spares you the leak and the cylinder head removal that goes with it, a job that costs far more than the part itself.