Reinforced head gasket Subaru Impreza WRX STI Forester Legacy 2.5 EJ25 EJ251 EJ253 EJ25D

The Subaru engines covered

The Subaru EJ25 is the 2.5-litre naturally aspirated boxer four-cylinder petrol engine of the Impreza, Legacy, Outback and Forester. This page covers its EJ25D, EJ251, EJ252 and EJ253 versions with the Athena part numbers.

On the EJ251 and EJ253, the original gasket carries part number 11044-AA521, or 11044-AA630 to 11044-AA633; on the EJ25D, 11044-AA471 or 11044-AA610. It is a multi-layer steel (MLS) gasket with a 100.5 mm bore, whose thickness Subaru does not publish. It is sized for standard combustion pressure: when that pressure rises, clamping load drops, the cylinder head lifts, and coolant passes into a cylinder.

Reinforced cylinder head gaskets

A cylinder head gasket does not create the seal on its own: it holds because the fasteners clamp the head against the block. A composite gasket is crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of the MLS gaskets withstands 517 N/mm², 5 times more, and springs back at every combustion.

This page offers the part numbers from Athena, a racing manufacturer, in two constructions: Cut Ring with stainless rings and MLS with Gas Stopper. Gasket bores run from 95 to 103.5 mm, covering the standard block as well as the rebored block, and thicknesses from 0.95 to 1.90 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: a boxer engine needs two gaskets, one per cylinder head.

Technical specifications

Gasket volume: the space the gasket leaves above each cylinder. It enlarges the combustion chamber, and the larger it is, the lower the compression ratio. Values compare with one another at identical gasket bore.

Preparation and fitting

Before fitting, check the flatness of the cylinder head and of the block with a straight edge and have them skimmed if necessary. These gaskets require no sealant: Athena supplies them already coated.

Technical guide

The two constructions offered

Athena Cut Ring A gasket in reinforced, asbestos-free MO115 material (steel core, aramid fibres, NBR binder), supplied with separate AISI 304 stainless steel rings around each cylinder. Athena also intends it for turbocharged or nitrous engines above 2 bar, with no machining of the cylinder head or block. A silicone bead reinforces the sealing of the water and oil passages. On fitting, the ring embossments face the cylinder head. Athena rates the MO115 material at 100 N/mm² at most: the stainless rings are what hold combustion.

Athena MLS Two to five layers of AISI 301 stainless steel, laser cut, stamped and riveted. Around each cylinder, a layer folded over on itself, the Gas Stopper, concentrates the clamping load on the combustion chamber, while beads seal the water and oil passages. The outer layers receive a two-coat finish: the first withstands high temperatures and fills small irregularities on the block and cylinder head, the second, silicone based, keeps the gasket from sticking on disassembly. This is Athena's all-steel choice, from a prepared naturally aspirated engine to moderate forced induction.

Choosing bore and thickness

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

Thickness. Subaru does not publish the thickness of the original gasket. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

With a thickness finer than that of the removed gasket, measure piston protrusion above the block with a dial gauge: it is what determines the remaining clearance between piston and cylinder head.

The brand

Athena S.p.A. is an Italian manufacturer of engine gaskets and components, with its head office and main plant in Alonte, in the Veneto region. The group also produces in Spain, the United States, Brazil and India. Its multi-layer gaskets are fitted as original equipment by car manufacturers, and its racing range is designed and made entirely 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 100.5 mm bore of the original gasket 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, 11044-AA631, 11044-AA632, 11044-AA633 (EJ251, EJ253) ; 11044-AA471, 11044-AA610 (EJ25D).

Important: the turbocharged EJ255 and EJ257 take other original gaskets (11044-AA670, 11044-AA770): they are not covered by this page.

Why switch to a reinforced gasket?

A reinforced gasket holds where the standard one reaches its limit: at higher combustion pressure it keeps its shape instead of being crushed.

As soon as power goes up, turbo, supercharger or raised compression ratio, the head gasket becomes the weak link of the top end. Switching to a reinforced gasket is then not a comfort but a safeguard: it is what prevents the failure and the head removal that comes with it.