Reinforced head gasket Subaru 2.5 EJ25 EJ255 EJ257

The Subaru engines covered

The EJ25 is a 2.5 litre flat-four petrol engine, naturally aspirated or, for the EJ255 and EJ257, turbocharged. It powers the Subaru Forester, Impreza, Legacy and Outback models in the table below. As it has two cylinder heads, it uses two head gaskets.

The original gaskets carry part numbers 11044-AA521 (EJ25) and 11044-AA670 (EJ255 and EJ257). Subaru publishes neither their bore, nor their thickness, nor their construction. They are sized for standard combustion pressure: when that rises, the clamping load drops, the head moves away from the block and coolant reaches a cylinder.

Reinforced cylinder head gaskets

Sealing does not rest on the gasket alone: the fasteners that clamp the head against the block play just as large a part. With the O-Ring, it is the stainless steel wire that seals the combustion chamber by itself, while the gasket body closes the water and oil circuits; with the MLS, the 301 stainless steel sheets withstand 517 N/mm², 5 times the stress at which a composite gasket compresses for good, and recover their shape with every combustion.

This page brings together three part numbers from Athena, an Italian maker of competition gaskets, in two constructions: the O-Ring, a steel sheet faced with aramid fibre and ringed with a stainless steel wire, with part numbers 330081R and 330131R, and the MLS, a stack of stainless steel sheets, with part number 338465R. All have a 101.3 mm bore, with thicknesses of 1 and 1.2 mm. As a rule, choose a gasket bore at least 0.5 mm larger than the cylinders. Each part number is sold individually; as the flat engine has two cylinder heads, two gaskets are needed.

Technical specifications

A gasket's volume is the space it leaves free above each piston. That space adds to the combustion chamber and lowers the compression ratio; here every gasket shares the same 101.3 mm bore, so only the thickness changes this volume.

Preparation and fitting

Before assembly, rest a straight edge on the head and then on the block to check their flatness, and have them resurfaced if necessary. Athena gaskets arrive ready to fit: do not add any sealing paste.

Technical guide

The two constructions offered

Athena O-Ring A steel sheet coated with asbestos-free aramid fibre, with a stainless steel compression ring crimped around each cylinder. The ring directs the clamping load onto a narrow band, exactly where the seal is made, while a silicone bead protects the water and oil passages. Because the combustion seal is carried by the ring and not by the gasket body alone, it is the construction that withstands the highest pressures in the range. It is supplied ready to fit, with no sealant to add.

Athena MLS A stack of two to five AISI 301 stainless steel sheets, laser cut, stamped and then riveted. A folded sheet encircles each cylinder and forms a Gas Stopper, which directs the clamping load towards the combustion chamber, while raised beads separate the water and oil circuits. The outer faces receive a coating: one layer withstands heat and fills small flaws in the block and head, a silicone layer stops the gasket from sticking at removal. It is Athena's all-steel solution, from the prepared naturally aspirated engine to moderate boost.

Choosing bore and thickness

Gasket bore. The Athena O-Ring (330081R and 330131R) and the Athena MLS (338465R) both come in 101.3 mm.

Thickness. Subaru publishes 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 the original thickness left unstated, use a dial gauge to measure how far the pistons stand proud of the block's gasket face, especially if you aim for 1 mm: it sets the clearance left between piston and head.

The brand

Athena S.p.A. is an Italian company specialising in gaskets and engine parts. Its head office and main plant are in Alonte, in the Veneto; it also has sites in Spain, the United States, Brazil and India. Several car makers fit its multi-layer gaskets as original equipment, and the entire 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 (101.3 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 (EJ25) and 11044-AA670 (EJ255, EJ257).

Important: the part numbers on this page only apply to the EJ25, EJ255 and EJ257 engines: note your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket reaches the end of its capacity, the reinforced gasket replaces it: under higher combustion pressure it does not compress and keeps its shape.

As soon as a turbo engine gets more boost pressure or a raised compression ratio, the head gasket becomes the first weak link of the top end. A reinforced gasket prevents the leak, and with it the head removal that follows, an operation far more costly than the part itself.