An ignition coil assembly adapted to be mounted directly in a plug hole of an internal combustion engine. The coil assembly includes a primary coil and a secondary coil. The spool upon which the secondary coil is wound is cylindrical in shape and has ramp structures provided at each end thereof. The ramps decrease voltage concentration between the secondary and primary windings, decrease the voltage gradient along the end wall of the secondary spool, and minimize adverse effects of gaps created during thermal expansion/contraction.