MGF head gasket replacement
There is a lot of urban myth about gasket failures on the K engine which is perpetuated at large on the various internet forums. The gasket itself is not really the problem. In a nutshell, here are the main issues:
- Many cases diagnosed as head gasket failure are a simple case of overheating and there is nothing wrong with the gasket itself. I have established this quite clearly on a number of such examples where the cause of overheating was rectified without replacing the gasket and the car performed without fault subsequently. Coolant loss/weeping from between the head and block is definitely due to the gasket, which should be replaced.
- In the early cars, the location between head and block was via plastic dowels. These aged and fractured, resulting in movement between the two components and gasket failure. Replacement head sets come with steels dowels which have eliminated this.
- Most other head gasket failures are a consequence of overheating due to some other cause. If overheating is noticed soon enough, then there is no downstream damage. However, in the case of the MGF because the engine and cooling system filler are behind the driver, there is no tell-tale trail of water/steam over the windscreen and if the gauge is not observed (as is often the case) then the overheating can become severe and the headgasket fails in response. If the car continues to be driven, the head will get so hot that it loses its tempering and goes soft. The only proper cures for this are to replace the head or to have it retempered, neither of which are cheap. An economical alternative which is reasonably effective if the head has not gone fully soft is to fit a steel shim between the gasket and head after refacing it. This restores the correct head thickness and also spreads the load from the fire ring. This repair is usually good but not up to the standard of a new head. Given the cost saving it is usually worth a try.
- Main cause of overheating is low coolant. Because the engine is in the rear and the radiator in the front, the cooling system is vulnerable to air locks which stop circulation to the radiator. The design of the thermostat actuation (which relies on bypass flow to open it) accentuates this as the bypass flow is relatively small. Later versions had a larger bypass flow which eliminated this.
- Thermostat failure is another cause of overheating, again the design of the thermostat actuation means less tolerance to minor faults.
- Repeat head gasket failures are invariably due to the head having gone soft the first time and that damage not having been rectified. If the head is soft then the fire rings on the gasket will have sunk into it, leaving an indentation. Many repairers in ignorance simply reface the head – if it is soft then the same thing will happen again quite quickly and the gasket has nothing to seal against.
In my experience and that of others who service a large number of these cars the original gasket is fine provided that the steel dowels are fitted and the head has not gone soft. However, some prefer a belts and braces approach and fit the revised gasket (which also comes complete with a head save shim should it be needed).