Apr 12, 2018 | GASKETS & SEALING


The EJ series 2.5l boxer engine has been powering many Subaru vehicles since 1996. With many different variations, they all share the potential for developing head gasket leaks. O.E.M. gaskets were composite or early generation MLS designs, both of which can be prone to leaks. Early gaskets tended to leak combustion gases into the cooling system or coolant internally into the oil. With coolant loss you have overheating. Later gaskets tended to develop external coolant and oil leaks. Initially notice by the smell of oil and coolant burning, leaks can also be seen at the head gasket joint. — There are many causes for these leaks: — Heat Cycles-Even though the motors are all aluminum blocks and heads and they expand at the same rate, these lightweight castings will flex. — Electrolysis-Can be a major cause of external coolant leaks. Corrosion at the battery cables and at the battery ground may be indicators of electrolysis in the cooling system which is causing the coolant to become corrosive, eroding gaskets and castings. — The Subaru Boxer Design-Flat and horizontally opposed means that oil remains in contact with the head gaskets instead of drain­ing back to the oil pan when the engine is shut off. Over time, this constant contact with oil can cause an inferior head gasket to deteriorate. — Abnormal Combustion-Detonation may cause head bolt stretch. Though detonation can occur in naturally aspirated engines, many owners of the turbocharged models will experience detonation and pre-ignition due to the increase cylinder pressures. This is often multiplied because the owners of the turbocharged Subaru modified the engines and increase boost to increase power. This creates even higher cylinder pressures and temperatures and increases the likelihood of abnormal combustion. And in extreme cases, the combination of increased temperature and pressure can cause O.E.M. head bolts to stretch. — The solutions start by replacing inferior gaskets with proven MLS gasket designs using multiple layers of harden stainless steel with embossed beads and special coatings. The ceiling beads act as springs to maintain sealing strength throughout heat cycles and extreme conditions. Coatings negate the possibility for excessive scrubbing as a castings expand and contract. As with any engine that uses torque-to-yield head bolts, new head bolts are recommended, even though factory repair manuals do not call for new bolts. Always follow the correct and most up to date torquing sequence and specifications when installing head bolts.

Engine Pro Technical Committee; with special thanks to Federal Mogul Corp.

April, 2018