Reinforced head gasket Subaru Forester Impreza Legacy Outback 2.5 EJ25

The Subaru engines covered

The EJ25 petrol engine, a 2.5 litre (2,457 cc) flat-four, powers the Subaru Forester, Impreza, Legacy and Outback listed in the compatibility table below.

The original gasket carries, depending on the version, the part numbers 11044-AA521, 11044-AA630 to 11044-AA633 (EJ251 and EJ253) or 11044-AA471 and 11044-AA610 (EJ25D). Subaru publishes neither its bore, its thickness nor its construction. Designed for the standard combustion pressure, it reaches its limit as soon as that pressure rises: the clamping load fades, one head lifts slightly and coolant finds its way into a cylinder.

Reinforced cylinder head gaskets

Sealing does not rest on the head gasket alone: it also depends on the fasteners that clamp each head to the block. A composite gasket is permanently crushed beyond 100 N/mm²; the 301 stainless steel of the MLX and MLS withstands 517 N/mm², 5 times more, and recovers its shape at every combustion.

This page presents part numbers from the American maker Cometic, a specialist in competition gaskets, in two constructions: the MLS, in 101 mm or 102 mm bore and 1.02 mm thickness, and the MLX with a chamber stopper, in 101 mm bore and five thicknesses from 0.91 to 1.30 mm. A flat engine has two cylinder heads, so two gaskets are needed.

Technical specifications

A gasket's volume is the free space it leaves above each cylinder. Added to the combustion chamber, it lowers the compression ratio; only gaskets of the same bore can be compared directly. The 102 mm MLS does not share the bore of the two 101 mm references, so compare its volume with care.

Preparation and fitting

Before fitting, run a straight edge over the gasket faces of each head and of the block, and have them resurfaced if needed. Cometic gaskets come coated: do not apply any sealant.

Technical guide

The two constructions offered

	Cometic MLS Several embossed stainless steel sheets joined by rivets. The outer sheets carry beads that surround each cylinder and each fluid passage: moderate clamping is enough to seal and the bore distorts little. The construction suits a tuned naturally aspirated engine as well as moderate boost.
	Cometic MLX The same base as the MLS, with a folded centre sheet forming a stopper around each combustion chamber. The stopper concentrates the clamping force around the bore and limits how far the gasket is crushed. It is designed for boosted engines and high combustion pressures.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered here in 101 mm (H1631SP6040SN) and 102 mm (H1631SP2040S), the MLX in 101 mm (C4578), in five thicknesses.

Thickness. Subaru publishes no thickness for the original gasket. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

As no original thickness is published, use a dial gauge to measure how far each piston protrudes above the block's gasket face, especially if resurfacing is planned: this reading shows the clearance left between piston and head.

The brand

Cometic Gasket, Inc. is an American company founded in 1989 in Concord, Ohio, that designs and makes engine gaskets and sealing parts. It built its name on custom gaskets cut to the bore and thickness required, all made in the United States 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 and 102 mm bores 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: 11044-AA521, 11044-AA630 to 11044-AA633, 11044-AA471 and 11044-AA610.

Important: the part numbers on this page suit only the EJ25 engine: check your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

Where the original gasket reaches its limit, the reinforced gasket keeps holding: under higher combustion pressure it keeps its shape and does not flatten.

As soon as power goes up, through a turbo, a supercharger or a raised compression ratio, the head gasket is the first part to give way in the top end. On a flat engine with two cylinder heads, a leak means removing the head concerned: a reinforced gasket spares you that job.