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Possible Blown Head Gasket??

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    #16
    I had my headgasket blow recently and got coolant / oil mixing and coolant getting into combustion chamber. Compression test showed 170-180 on all cylinders.

    Compression is not a direct indicator of a blown headgasket

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      #17
      TMI on M42 coolant system bleeding woes

      As far as bleeding the coolant system goes, it SHOULD be insanely easy. The procedure gets blown a little out of proportion as to how complicated it is due to an age-related quirk the thermostat bypass setup. If the bypass is plugged, then yeah bleeding is more involved and you have to do stuff like jack the front up and let it fully heat. When the bypass is clear, the system will be 95% bled within 10 seconds of starting the engine.

      Most thermostats have a little hole in them to allow air to bleed through when they are cold/closed. The M42's does not have this hole, and instead there is a groove cast into the aluminum thermostat housing/cover. This groove bridges the housing's inlet and outlet sides. It is also somewhat prone to becoming clogged with crap over the decades (calcium & lime from tap water, "fix a leak" additives, RTV, gasket sealer, etc). If the groove is clear, most of the air will be out of the system very rapidly. In fact, if this car is new (to you), I would just plan to replace the thermostat (88°C is the correct version) and make sure that the groove in the housing is clear. You really only need the thermostat and the housing gasket.

      Do not use RTV sealer on the gasket. If you feel that you must use sealer, use some Permatex High Tack Spray-A-Gasket, and apply only a thin coating to the gasket so that it doesn't squeeze out and plug the passage. Also be careful in torquing down the 4 bolts that secure the housing...if you do it wrong, you will crack the housing. See the graphic below (really, this applies to just about any assembly).



      And yeah, clear the cat. Having that plugged is very bad juju.

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