Reinforced head gasket Subaru 2.5 EJ25D EJ251 EJ252 EJ253 EJ255 EJ257

The Subaru engines covered

The EJ25D, EJ251, EJ252, EJ253, EJ255 and EJ257 are 2.5 litre petrol engines, turbocharged on the EJ255 and EJ257. They power the Subaru Impreza, Forester, Legacy, Outback, Baja, Exiga and WRX models and the Saab 9-2X listed in the compatibility table below.

The original gaskets carry part numbers 11044-AA471 and 11044-AA610 on the EJ25D, 11044-AA521 and 11044-AA630 to 11044-AA633 on the EJ251, EJ252 and EJ253, then 11044-AA670 and 11044-AA770 on the EJ255 and EJ257; the later EJ257 carries part number 11044-AA910. Subaru publishes neither their bore, nor their thickness, nor their construction. Each engine takes one cylinder head gasket, sized for the standard combustion pressure; when that pressure climbs, the clamping force eases, the head lifts slightly and coolant seeps into a cylinder.

Reinforced cylinder head gaskets

Sealing does not rest on the gasket alone but also on the fasteners that press the head onto the block. Beyond 100 N/mm², a composite gasket compresses for good; the 301 stainless steel of MLS gaskets takes 517 N/mm², 5 times more, and recovers its shape at every combustion. Also inspect the head bolts: replace any bolt that is stretched or damaged. Do this for the head bolts of your Subaru (EJ25D/EJ251/EJ252/EJ253/EJ255/EJ257).

This page brings together 16 Cometic part numbers, from the American maker of competition gaskets, in two constructions: MLS, multi-layer steel, with 14 C4264 part numbers in a 100 mm bore, from 0.69 to 3.05 mm, and MLX, with a chamber stopper, with 2 C4578 part numbers in a 101 mm bore, from 1.02 to 1.12 mm. An engine takes only one gasket: order a single part number.

Technical specifications

A gasket's volume is the free space it leaves above each cylinder; it adds to the combustion chamber and lowers the compression ratio. Compare only gaskets of the same bore: 100 or 101 mm here.

Preparation and fitting

Before reassembly, check the flatness of the head and block mating faces with a straightedge and have them resurfaced if needed. Cometic gaskets arrive already coated: do not apply any sealing paste. Remember to clean the gasket faces carefully before assembly. Do this on the head and on the block of your Subaru (EJ25D/EJ251/EJ252/EJ253/EJ255/EJ257).

Technical guide

The two constructions offered

Cometic MLS Several stainless steel sheets, stamped and riveted together. Each outer sheet carries active beads tracing the outline of every cylinder and fluid passage. A moderate clamping load is enough to seal, and the bore distorts very little. This versatile gasket suits anything from a tuned naturally aspirated engine to light boost. The C4264 part numbers on this page are offered in this construction.

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. Part numbers C4578 on this page are offered in this construction.

Choosing bore and thickness

Gasket bore. The Cometic MLS (C4264) is offered in 100 mm; the Cometic MLX (C4578) is offered in 101 mm.

Thickness. No thickness is published for the original gasket 11044-AA471 ; 11044-AA610 ; 11044-AA521 ; 11044-AA630 ; 11044-AA631 ; 11044-AA632 ; 11044-AA633 ; 11044-AA670 ; 11044-AA770 ; 11044-AA910. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

Before opting for a thickness below the original, measure with a dial gauge how far the pistons protrude above the block deck: it shows the clearance remaining between piston and head. On these engines, take that measurement at each piston.

The brand

Cometic Gasket, Inc. is an American company founded in 1989 and based in Concord, Ohio, which designs and builds gaskets and sealing parts for engines. It earned its name with made-to-measure gaskets, cut to the bore and thickness required, all made in the United States from American materials. Its part numbers for these Subaru engines are offered here in 100 mm (MLS) and in 101 mm (MLX).

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 and 101 mm bores of these gaskets 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-AA471 and 11044-AA610 for the EJ25D; 11044-AA521 and 11044-AA630 to 11044-AA633 for the EJ251, EJ252 and EJ253; 11044-AA670 and 11044-AA770 for the EJ255 and EJ257; 11044-AA910 for the later EJ257.

Important: the part numbers on this page suit only the EJ25D, EJ251, EJ252, EJ253, EJ255 and EJ257 engines: check your Subaru's engine code before ordering.

Why switch to a reinforced gasket?

When the original gasket is no longer enough, the reinforced gasket replaces it: under higher combustion pressure it keeps its shape instead of collapsing. For the EJ25D, EJ251, EJ252, EJ253, EJ255 and EJ257 on this page, it is the answer as soon as the engine goes beyond its original output.

Whether you add a turbo, raise the compression ratio or rev higher with livelier cams, the cylinder head gasket becomes the weak link of a high-output engine. A reinforced gasket prevents the leak and the head removal that goes with it, far costlier than the gasket. On the EJ25D, EJ251, EJ252, EJ253, EJ255 and EJ257 engines, the reinforced gasket is the first item to address once you go beyond the original output.