Reinforced cylinder head gasket Jaguar V8 4.0L 4.2L AJ27 AJ33

The Jaguar engines covered

The Jaguar AJ27, AJ27S, AJ33 and AJ34 petrol engines, V8s of 4.0 and 4.2 litres, power several Jaguar models (XK8, XK, XJ, S-Type and Vanden Plas) detailed below. Each has two cylinder heads: one gasket is needed per cylinder bank, two per engine, one right and one left.

The original gasket carries part number NCC2540BC for the right bank and NCC2541BC for the left bank, valid for the 4.0 L up to engine 0008210147 (XK8). Jaguar publishes neither the bore, nor the thickness, nor the construction. It is sized for the standard pressure: when pressure rises, the clamping load drops, the head lifts and coolant reaches a cylinder.

Reinforced cylinder head gaskets

The head gasket does not seal on its own: the fasteners that clamp the head to the block matter just as much. A composite gasket settles for good beyond 100 N/mm²; the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more, and recovers its shape at every combustion.

This page gathers the part numbers of Cometic, an American maker of competition gaskets, in a single construction: MLS, multilayer steel. The gaskets come in a 93 mm bore, in two versions, one per cylinder bank, and in thicknesses from 0.69 to 3.56 mm, enough to adjust the compression ratio without touching the pistons or the cylinder 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 and covers one cylinder bank only: order one for the right bank and one for the left bank.

Technical specifications

Gasket volume is the open space remaining above each cylinder. It adds to the combustion chamber and lowers the compression ratio in proportion. Compare only gaskets of the same bore.

Preparation and fitting

Before installing the gasket, check with a straightedge that the gasket faces of the head and the block are flat, and have the one that needs it resurfaced. Add no sealant: Cometic gaskets are delivered already coated.

Technical guide

The construction offered

MLS Cometic Several pressed stainless steel sheets fixed together with rivets. The outer sheets carry active beads around each cylinder and each fluid passage. Moderate clamping is enough to seal and the bore distorts very little. A versatile construction, from the tuned naturally aspirated engine to moderate forced induction.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in 93 mm for the right bank (C4377) and for the left bank (C4376).

Thickness. Jaguar publishes 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 a thickness below the standard one, start by reading with a dial gauge how far the pistons protrude above the block face: it sets the clearance left between piston and head.

The brand

Cometic Gasket, Inc. is an American company based in Concord, Ohio, founded in 1989. It designs gaskets and sealing solutions for engines and made its name with the made-to-measure gasket, supplied at the bore and thickness required. All production stays in the United States, with 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 bore size of the original gasket, a figure Jaguar does not release 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: NCC2540BC, NCC2541BC.

Important: the part numbers on this page match the Jaguar AJ27, AJ27S, AJ33 and AJ34 engines listed above: note your vehicle's engine code, then identify the cylinder bank concerned before ordering.

Why switch to a reinforced gasket?

Once the standard gasket reaches its limit, the reinforced gasket steps in: at higher combustion pressure, it keeps its shape instead of settling.

With a turbo, a supercharger or a raised compression ratio, the cylinder head is under greater load and the gasket becomes the weakest part of the top end. The reinforced gasket prevents a gas or coolant leak, and the head removal that follows, with the car off the road.