O-RING

Subaru Impreza EJ Series Engine Head Gasket Replacement

Project Summary:

Difficulty Level (1-5)	4 - Advanced/Intermediate
Time Required	8-16 hours
Cost	Starting at $100 for head gaskets. Costs go up quickly with o-ringing and performance parts.
Recommendation	Required if the head gaskets are blown. Highly recommended if you have added a turbo to a non-turbo pre-1999 engine.

This installation will provide tips to the steps required to replace a set of head gaskets on a Subaru EJ series engine. Due to the horizontally-opposed (flat-4) design, there are special challenges to doing a head gasket replacement as compared to a normal inline 4-cylinder or V-type engine.

Note: these instructions are intended for someone with extensive automotive repair experience and knowledge. Many details have been omitted, as they can be found in other literature such as Haynes' excellent Subaru Legacy Automotive Repair Manual (1990 through 1998). Missing details include: engine removal, locations of every nut and bolt, all intake and coolant system plumbing details, and wiring details. However, I have included pictures wherever possible to aid presentation and to provide helpful hints.

Materials Required:

2 Head gaskets, model-year specific
Coolant/antifreeze (Prestone, etc.)
Motor oil
Spark plugs (optional)
Timing belt (optional)
Accessory belts (optional)
Shop rags

Tools Required:

10mm, 12mm, 14mm, 17mm, 21mm socket (ratchet) wrenches
22mm crankcase pulley socket wrench
14mm deep socket wrench
12" or longer breaker bar
4" and 6" socket wrench extensions (optional)
10mm, 12mm, 14mm, 17mm open-ended wrenches
2-ton floor jack(s) or equivalent
2-ton jackstand(s) or equivalent
Oil/fluid catch pan or suitable container
Pliers
Engine hoist or suitable chain hoist
Engine stand (optional)
Small flat-blade screwdriver (optional)

Let's Start

Remove engine from car, and place on engine stand or suitable work surface so you can apply a lot of torque without damaging the oil pan and other engine parts.

Remove heads using untorquing sequence as shown in the Subaru Service Manual or Haynes Manual. I have provided a copy below. If your head gaskets have been blown for a while, be warned that water may have escaped into the head bolts and rusted them to the engine block.

If your head gaskets are blown, you should expect to see slight warping of the gasket's metal layers (yellow arrow), and tell-tale marks of combustion gases seeping past the fire ring (green arrow) and graphite filler (red arrow) into the coolant passages. This is what causes purging of the coolant into the reservoir when the head gaskets are blown.

Overall, the gaskets will also stretch (direction of yellow arrows). I do not know the exact reason why, but it should be safe to say that the open "decks" in these areas are "walking", whereas the solid metal connections at the left, middle, and right of the cylinders do not allow any movement of the cylinder walls in those areas.

Here is a selection of EJ series head gaskets:

EJ22T SOHC, 1991-1993 Legacy Turbo, 8:1 compression ratio. Gasket is approximately 1.5mm thick installed. Made of two sheets of steel with a graphite filler.

EJ25 DOHC, 1998 2.5RS, 9.7:1 compression ratio. Notorious for blown head gaskets even in normally aspirated form. Gasket is approximately 1.5mm thick installed. Made of a sandwich of layers of steel, graphite, and an unknown material. Sandwich is held together with small rivets prior to installation in the engine.

EJ25 SOHC, 2000+ 2.5RS, 10:1 compression ratio. Very few reported problems of blown head gaskets. Gasket is approximately 0.6mm thick installed. Made of a single sheet of steel.

Here is a closeup look at the multi-layer construction of the original EJ25 DOHC head gasket. Subaru has since replaced this design with a newer one. Although I don't have a picture, I can tell you that the newer design replaces the outermost graphite layers with solid steel. The fire ring is also much more heavily reinforced with steel than before.

Here is a quick diagram of what o-ringing is all about. The idea is mainly to provide a much higher clamping force on the head gaskets using a small metal ring.

This is what an o-ringed head looks like.

Not much here, except for the two thin rings added to the head surface. This excellent example of craftsmanship was performed by Charlie North of Function First Motorsports in East Wallingford, VT. You should be able to find a highly skilled machinist to do a similar job.

For your convenience, I have included the cylinder head torquing sequence from the Subaru Service Manual. It should work for all EJ series 4-cylinder engines.

The torquing sequence seems rather strange, and when it asks to back all bolts off by 180°, don't be surprised to find that the bolts will be loose!

It is up to you whether to re-use the old head bolts or replace them with new ones. I've done both, and the old ones didn't feel any different than the new ones except that the rusted bolts required a lot more torque to break loose during removal.

Cylinder Head Torquing Sequence (1 of 2)

Standard disclaimer: This information is being provided as an educational and informational service for the Subaru community. I do not make any warranties, but I will be happy to help with questions about these modifications. Please proceed at your own risk!

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