If there’s no compressibility in the gasket if the surfaces are not 100% flat, part of the surface will have less sealing pressure than the rest, and a leak can occur. The surfaces either side of the gasket must be perfectly smooth and perfectly flat. Well, the downside is Steel shim gaskets are not at all tolerant of surface imperfections. As they don’t compress over time, re-tensioning was also a thing of the past. The style of the gasket is exceptionally robust and almost impossible to blow as it’s harder than the surrounding aluminium cylinder block and head. This shim style of head gasket started being phased in around the mid-’90s on Subaru’s and by 2000 was across the board. Sometimes referred to as MLS gaskets they are a thin stainless steel shim or layers of thin steel shims riveted together, sometimes with a sealing coating.īetter gasket technology and better surface finishing machinery and assembly process have made the compressible nature of the fibre composite head gasket redundant.
The loss of cylinder compression usually causes an engine to run on less than all its cylinders.Ĭomposite style head gaskets have almost completely vanished from modern engines, Subaru along with most manufacturers have abandoned them in favour of metal shim style gaskets. Also, it can allow coolant to enter the cylinder or mix with the oil. The term ‘blown head gasket’ refers to a hole blown through the sealing material to allow combustion pressures to escape.
If the metal ring had any breach of sealing, the fibre material backing it up would ‘blow’ out due to the extreme pressure which would result in a catastrophic failure. They would sometimes need to be re-tensioned as the material compressed over time. These were known as composite gaskets, and they had their limitations. So, why do we hear so much about head gaskets and why is the year so important?Įarly engines had a head gasket that was made from a compressible fibre material with metal rings around the important parts to contain the extreme pressures of combustion. Before 2000 it could happen, but it still wasn’t a common problem. The fact is, rarely does the post Year 2000 Subaru blow the head gasket. The term ‘blown head gasket’ seems to be regularly used amongst Subaru owners and enthusiast chat groups. It seems daily we read about or directly hear about someone disappointed with their Subaru’s ‘weak’ head gaskets.
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