Reinforced head gasket Jaguar E-Type Mark X 420 XJ6 4.2 XK 7L

The Jaguar engines covered

The 4.2 litre (4,235 cc) XK petrol straight-six engines, codes 7L and XK 4.2, with twin overhead camshafts, power the Jaguar E-Type, Mark X, 420, 420G and XJ6 models detailed below.

The original gasket is listed as NXD2520AA (XJ6 4.2 litre); the manufacturer publishes neither its bore, nor its construction, nor its thickness. This gasket is sized for the standard combustion pressure: when that rises, the head loses clamping force, lifts off the block and lets coolant reach a cylinder.

Reinforced cylinder head gaskets

The gasket does not guarantee the seal on its own: the fasteners that clamp the head onto the block weigh just as much. CFM-20 combines a galvanized steel core with a soft asbestos-free compound that makes up for small irregularities in the gasket faces. On these Jaguars, torquing in several passes keeps the pressure even.

This page offers the part numbers from the American maker Cometic, a specialist in racing gaskets, in one construction only: the CFM-20. Their bore is 93 mm and their thicknesses 1.09 mm and 1.50 mm. Each part number is sold individually: one gasket per engine.

Technical specifications

A gasket's volume is the space it leaves free above each cylinder. That space adds to the combustion chamber and lowers the compression ratio; only gaskets of the same bore are comparable. Here, both gaskets have a 93 mm bore.

Preparation and fitting

Before fitting, run a straight edge over the head and the block to check their flatness, and have either one resurfaced if necessary. Cometic checks the flatness of each of its CFM-20 gaskets. This check holds for the 4.2 litre XK as for any other engine.

Technical guide

The construction offered

Choosing bore and thickness

Gasket bore. The Cometic CFM-20 gasket is offered in 93 mm (C4338-043 and C4338-059), with no other bore.

Thickness. The manufacturer gives no thickness for the original gasket of these engines. 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, measure with a dial gauge how far the pistons stand out of the block's gasket face, above all if the removed gasket is thinner than 1.09 mm: this measurement gives the clearance left between piston and head.

The brand

Cometic Gasket, Inc. is an American manufacturer of gaskets and sealing solutions for engines, founded in 1989 in Concord, Ohio. Its reputation rests on the made-to-measure gasket, cut to the bore and thickness requested; all production takes place in the United States, from American raw materials. The part numbers C4338-043 and C4338-059 are its own.

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 93 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: NXD2520AA.

Important: the part numbers on this page suit the Jaguar 7L and XK 4.2 engines of 4.2 litres: check the engine code of your vehicle before ordering. The 8LC code is not dealt with on this page: enquire before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches its limits, this CFM-20 gasket takes over: the steel core keeps its shape under combustion pressure, while the soft compound on both faces follows the small flaws of the head and block. The 4.2 litre XK finds here two gaskets with a 93 mm bore.

On an engine driven to its limit, the head gasket is the first part to fail, the weak point of the top end. A reinforced gasket prevents the leak, whereas the head removal that follows, a long and delicate job, costs much more than the gasket. This holds for the 4.2 litre XK as for any hard-driven engine.