Abstract: An ignition coil, for a spark ignition system, comprising a hollow casing closed at one end by a base and closed at its opposite end by an electrically insulating cap carrying at least the high voltage output terminal of the coil. Primary and secondary windings are housed within the casing, and an annular resilient sealing gasket is trapped between mutually presented faces of the casing and the cap to seal the interface of the casing and the cap. Adjacent said gasket the casing has at least one localized region so shaped as to provide a weak point in the seal effected by the gasket between the casing and the cap, whereby when the pressure within the casing exceeds a predetermined value the sealing action at said weak point fails so relieving the excess pressure in the casing.