Reinforced cylinder head gasket Subaru Impreza Forester Legacy Outback 2.5 16V EJ25 EJ251 EJ252 EJ253 EJ25D

The Subaru engines covered

The Subaru engines on this page, of 2.5 litres capacity, with 16 valves, carry the codes EJ25, EJ251, EJ252, EJ253 and EJ25D. They power the Subaru Impreza, Forester, Legacy, Outback, Baja and Exiga in the table below, since 1995. With one cylinder head per bank, the gasket is sold individually: one is needed per side.

The original gasket carries the part number 11044-AA521. Subaru publishes neither its bore, nor its thickness, nor its construction. It is sized for standard combustion pressure: when that rises, the clamping load drops, the head lifts and coolant reaches a cylinder.

Reinforced cylinder head gaskets

Holding power does not rest on the gasket alone: it also relies on the fasteners that clamp the head against the block. A composite gasket settles with no return beyond 100 N/mm², whereas the 301 stainless steel of the MLS withstands 517 N/mm², 5 times more, and relaxes with each combustion. With the O-Ring, the stainless wire is enough to isolate the combustion chamber, and the gasket body closes the water and oil circuits.

This page brings together the part numbers of the racing manufacturer Athena, in two constructions: the stainless-ring O-Ring and the multilayer MLS. Gasket bores run from 95 to 103.5 mm and thicknesses from 0.95 to 1.9 mm, enough to tune the compression ratio without touching the pistons or the 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, one gasket per engine.

Technical specifications

The volume of a gasket is the space it leaves free above each cylinder. This volume is added to that of the combustion chamber: the more it grows, the more the compression ratio falls. Only compare gaskets of the same bore.

Preparation and fitting

Check the flatness of the head and block with a straightedge before fitting, and have them skimmed if necessary. Do not add sealant: Athena delivers its gaskets already coated.

Technical guide

The two constructions offered

O-Ring / Cut Ring Athena A steel sheet dressed with asbestos-free aramid fibre, around which a stainless compression wire is crimped at each cylinder. The wire transfers the clamping load to a thin band, at the exact spot where the seal is won or lost, and a silicone bead preserves the water and oil passages. This construction copes best with extreme combustion pressures and suits the most heavily loaded engines.

Athena MLS Between two and five sheets of AISI 301 stainless steel, laser-cut, stamped and riveted together. One of them is folded around each cylinder to form a Gas Stopper that steers the clamping load towards the combustion chamber, while beads close the water and oil circuits. The outer faces receive a coating: one layer withstands heat and takes up small flaws in block and head, a silicone layer keeps the gasket from sticking at removal. It is Athena's all-steel answer, suited from the tuned naturally aspirated engine to moderate boost.

Choosing bore and thickness

Gasket bore. The Athena O-Ring comes in 95, 99, 100, 101.3 and 103.5 mm, the Athena MLS in 100, 101 and 102.3 mm.

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

Subaru gives no thickness for the original gasket, so use a dial gauge to read how high the pistons rise above the block deck: the value gives the clearance left between piston and head, above all with the thinnest gasket of the range.

The brand

Athena S.p.A. is an Italian maker of gaskets and engine parts, based in Alonte, in the Veneto, where its head office and main plant stand together. It also has sites in Spain, the United States, Brazil and India. Its multilayer gaskets are fitted as original equipment by car manufacturers, and its whole competition range is designed and made in Italy.

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 of the gaskets offered (95 to 103.5 mm) 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.

Important: the gaskets on this page are listed for the Subaru models in the table, since 1995. For any other vehicle, note the engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket is outmatched, the reinforced one takes over: under a stronger combustion pressure it keeps its shape and does not settle.

On a built engine, a rise in boost pressure or compression ratio tests the head gasket first. A reinforced gasket rules out the leak, and with it the removal of the cylinder head: an operation whose cost is out of all proportion to that of the part.