Engine Design

subaru head gasketBefore getting involved with what the head gasket is and why is it prone to failure, let’s first take a look into the engine design of the Subaru “Boxer” Engine.

Subaru build’s both flat-four and flat-six engines. Built on a horizontal plane, the “Boxer” engine is so called due to the direction of the piston movement, which resembles that of a boxer’s fists. However, the key benefits of this design are that half of the engine’s pistons move in the opposite direction at one time. Half of the pistons lay to the east, and half of the pistons lay to the west.

This offers multiple benefits, including an increase in vehicle handling and stability, and as a result, safety. Not only does the lower center of gravity of the “Boxer” engine offer a smaller roll angle, the smaller mass of the engine, in particular length, is much shorter compared to a conventional “in line” or “V” engine layout. This contributes to an improvement in weight distribution, which in turn helps the car handle better.

Subaru “Boxer” engines are relatively small and lightweight compared to their conventional counterparts. Subaru crankcase and cylinder heads are all of aluminium construction, which significantly contributes to weight reduction. This helps to improve power, but also helps handling and stability.

Head Gaskets


Image used for demonstration purposes only

One disadvantage of the “Boxer” engine is that there are two cylinder heads. These are separated from the engine block by a gasket thats purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders.

Coolant and engine oil are pumped through the engine block by pumps, and their jobs are to keep the engine cool and lubricated, respectively. One issue that many Subaru owner’s will face is the dreaded “head gasket failure”. This occurs as a result of the head gasket failing at one or multiple points, allowing engine oil or coolant to pass through the crankcase and into the cylinder internally, or externally causing a leak. Another form of gasket failure is when the gasket allows oil and coolant to mix through through their respective passages in the engine block, which will result in coolant in your oil or vice versa.

Types of head gasket failure

Engine misfire – This generally occurs as a result of a head gasket failing between two cylinders, resulting in compression from one cylinder leaking into another. This will result in rough idling.

Overheating – This occurs as a result of failure between the combustion chamber and the cooling system, resulting in loss of coolant and overheating. For example, a spirited drive through the mountains will result in a build up of heat in the engine block, allowing pressure to build up in the cooling system as combustion gasses pass through from the combustion chamber. This will result in overheating and increases the likelihood of severely damaging both your engine and cooling system components. Other symptoms include white smoke from the exhaust, significant loss of coolant with no visible leaks, and bubbles in the radiator or coolant overlow tank.

Oil in the coolant and coolant in the oil – Head gaskets can fail between the lubrication system and the coolant passages, resulting in oil in the coolant or coolant in the oil. As coolant combines with engine oil, it will prevent the oil from lubricating the system as its ability to lubricate and viscosity are changes. This is often difficult to diagnose. Oil in the coolant can be seen much easier, as a visible sludge can be found in your coolant overflow bottle or at the top of your radiator.

An external leak – Oil leaking from your car can be a symptom of many different issues, ranging from a leaking oil filter, oil drain plug, oil filler cap, turbo seal, or rocker cover gaskets. In a Subaru, visible leaks from the head gasket are often 100% confirmation that there is a failing head gasket

Diagnosis and Repair

At Rockstar Automotive, we are fully equipped to diagnose and repair head gasket failures on all makes and models of Subaru.

Models Affected

2.5 litre Subaru affected models include:
1997 – Current: Subaru Outback
1999 – 2010: Subaru Liberty
2003 – Current: Subaru Forester
2003 – 2005: Subaru Impreza

If not diagnosed and repaired quickly, Subaru head gasket failure can lead to rusting of the cast iron bore, engine failure and seizure, and permanent damage to the cooling system. If you believe your car is suffering, get in touch with Rockstar Automotive today to find out how we can help.