Reinforced cylinder head gasket Jaguar XK120 XK140 XK150 D-Type Mark 2 3.4 XK 3.4

The Jaguar engines covered

Code XK 3.4: the Jaguar 3.4 litre (3,442 cc) petrol in-line sixes, driven by twin overhead camshafts, power the XK120, XK140, XK150, C-Type, D-Type, XKSS, Mark VII, VIII and IX, Mark 1, Mark 2 and 340 listed below.

The gasket fitted at the factory carries the number EAC8349. The manufacturer states neither its bore, nor its construction, nor its thickness. It is sized for the standard combustion pressure: beyond it, the head loses clamping, lifts and coolant enters a cylinder.

Reinforced cylinder head gaskets

The seal does not rest on the gasket alone: the fasteners that clamp the head to the block weigh as much as it does. Beyond 100 N/mm², a composite gasket settles with no recovery; the 301 stainless steel of MLS gaskets takes 517 N/mm², 5 times more, and regains its shape at every combustion.

This page gathers the part numbers of Cometic, an American maker of competition gaskets, in two constructions: the multi-layer stainless steel MLS and the CFM-20, with a steel core covered by an asbestos-free compound. The bores are 83, 84, 84.5 and 85 mm and the thicknesses run from 0.69 to 3.56 mm. Each part number is sold individually: one gasket per engine.

Technical specifications

A gasket's volume is the void it leaves above each cylinder. That void adds to the combustion chamber and reduces the compression ratio, all the more when the gasket is thick and the bore large. Compare these values only between gaskets of the same bore.

Preparation and fitting

Before assembly, check the flatness of the head and block faces with a straightedge, and have any that need it machined. Use no sealing paste: Cometic gaskets are supplied coated.

Technical guide

The two constructions offered

	CFM-20 Cometic A galvanized steel core covered on both faces with a soft asbestos-free compound. This laminated construction, part metal and part fibre, combines the strength of steel with the softness of the compound. It is offered here in two bores, 83 and 85 mm, and in two thicknesses, 1.09 and 1.50 mm.
	Cometic MLS Five stainless steel sheets, embossed, stacked and riveted. The outer sheets draw active beads around the cylinders and the fluid passages. A measured tightening torque is enough to seal, without distorting the bore. This gasket suits the tuned naturally aspirated engine as well as moderate forced induction.

Choosing bore and thickness

Gasket bore. Cometic MLS is offered in 83 mm (C4281), in 85 mm (C4303), in 84.5 mm (C14108-050) and in 84 mm (C14110-050); the CFM-20 in 83 mm (C4302) and in 85 mm (C4304).

Thickness. The manufacturer gives no thickness for the original gasket of the 3.4 litre XK. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

For a thickness below the original, first read with a dial gauge how far the pistons stand proud of the block deck: this measurement gives the clearance remaining between piston and head.

The brand

Cometic Gasket, Inc. is an American company created in 1989 in Concord, Ohio, that designs engine gaskets and sealing solutions. It made its name with the made-to-measure gasket, cut to the bore and thickness requested. All production stays in the United States, with 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 83, 84, 84.5 or 85 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: EAC8349.

Important: the part numbers on this page suit the manufacturer's 3.4 litre Jaguars from 1949 to 1970 with the XK6 engine: check your engine's bore before ordering, 83, 84, 84.5 or 85 mm. The C4303 and C4304 part numbers, in 85 mm, apply only to the Mark VII, Mark VIII, XK120, XK140 and XK150.

Why switch to a reinforced gasket?

When the original gasket reaches its limits, the reinforced gasket replaces it: under higher combustion pressure, it keeps its geometry and does not sag.

With a turbo, a supercharger or a higher compression ratio, the head works harder and the gasket becomes the weak point of the top end. A reinforced gasket prevents the gas or coolant leak and the head removal that follows, with the car off the road.