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The ring nuts, with castellations around their edges, are usually found to be chewed up, after idiots have attacked them with hammer and screwdriver in a vain attempt to overtighten them to stop a water leak that is actually rarely due to the ring nuts not being tight enough, and in reality caused by a failure of the sealing washers situated between the plug ferrules and the alloy water-dome casting. These used to be a copper and asbestos sandwich, but asbestos is now banned, and much better alternatives can be used, together with silicone mastic sealant. The flanges on the plug ferrules need to be in good condition if a good watertight seal is to be made, and you also need a good ‘nip’ on the outer sealing rings between the water dome and the top of the cylinders. The way I do it is to put the dome in place with the chosen sealing washer in place on the plug ferrules, but NOTHING on the outer joint. Tighten down the ring nuts with the special tool, (easily made out of an old tube spanner, or length of steel tubing), but not too tight. Then measure the gap between the outer edge of the water dome and the top of the cylinders. Add about ten thou” to that measurement, and that is the thickness of sealing rings that you need. In other words, you are trying to get the same amount of ‘nip’ on both the outer rings and the plug ferrule washers. Finally assemble it with all the packings in place, aided by silicone mastic sealant, and tighten down firmly. Leave at least overnight for the silicone to cure before starting the engine. Before assembly do check carefully that there are no radial cracks in the aluminium around the inner holes of the water dome, or you will never achieve a water-tight joint.