Reinforced head gasket Subaru 2.5 EJ253 EJ255

The Subaru engines covered

The EJ253 and EJ255 petrol engines of 2.5 litres power the Subaru Forester, Impreza, Legacy, Outback, Baja, Exiga and WRX as well as the Saab 9-2X, detailed below.

Two original part numbers exist: 11044-AA521 for the EJ253 and 11044-AA670 for the EJ255. Subaru publishes neither their bore, nor their thickness, nor their construction. Cut for the standard combustion pressure, they give up as soon as it climbs: the clamping force weakens, the cylinder head lifts a little off the block and coolant finds its way into a cylinder.

Reinforced cylinder head gaskets

Sealing does not rest on the gasket alone: the fasteners that press the cylinder head against the block provide half of it. A composite gasket is crushed for good beyond 100 N/mm² of stress; the 301 stainless steel of MLX gaskets takes 517 N/mm², 5 times more, and regains its shape at every combustion.

This page covers the part number of the American maker Cometic, a specialist in competition gaskets, in a single construction: MLX. Its bore is 101 mm and its thickness 0.81 mm. The gasket is always slightly wider than the cylinder: by convention, at least 0.5 mm more on the bore. It is sold individually: one gasket per engine.

Technical specifications

A gasket's volume is the room left free above each cylinder. It adds to the combustion chamber and reduces the compression ratio; a comparison only makes sense between gaskets of the same bore.

Preparation and fitting

Before fitting, check with a straight edge that the cylinder head and block are flat, and have them machined if needed. Cometic supplies this gasket coated: avoid any sealing paste.

Technical guide

The construction offered

Cometic MLX Several stainless steel layers stamped and riveted together, with a folded centre layer forming a stopper around the combustion chamber, and a high-temperature coating. Local clamping rises without the bore distortion increasing. It is the strongest of the all-steel gaskets, to be chosen for heavy boost.

Choosing bore and thickness

Gasket bore. The Cometic MLX comes in 101 mm (C4622-032), with no other bore.

Thickness. Subaru gives no thickness for the original gaskets. 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 protrude above the block face, especially if the removed gasket is thinner than 0.81 mm: this measurement shows the clearance left between piston and cylinder head.

The brand

Based in Concord, Ohio, and founded in 1989, Cometic Gasket, Inc. designs and makes gaskets and sealing parts for engines. It made its mark with the made-to-measure gasket, cut to the bore and thickness wanted, from 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 101 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: 11044-AA521 and 11044-AA670.

Important: the part number on this page suits the EJ253 and EJ255 engines: check your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches its limits, the reinforced gasket takes over: even under higher combustion pressure, it does not settle and keeps its geometry.

As soon as a turbo, a supercharger or a higher compression ratio comes into play, the head gasket becomes the weak point of the top end. A reinforced gasket rules out the leak and the cylinder head removal that follows, an intervention markedly dearer than the part.