Silicone sealant tends to lift away over time (you only have to look at what eventually happens to it round the edge of your bathtub). The lifting and peeling will occur even quicker on the HT lead connections because of the heat from the engine, vibrations and the flex in the lead. Once it starts lifting it allows water to get drawn under it by capillary action and, if it does get in there, gets trapped and can make problems worse and everything harder to dry out.

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