The 3055 is being used as a switch, not an amplifier. The circuit you show is very poorly designed. The 10k resistor between pin 3 of the 555 and the base of the transistor is much too high in value. This causes the base current to the 3055 to be much too low when it is turned on, preventing the 3055 from turning on hard enough to minimise power dissipation when it is turned on. It needs to be mounted on a heatsink.
You seem to be confused about how to mount a TO3 NPN power transistor. You need to buy a "mounting kit" consisting of a mica washer, teflon bushings, which allow you to mount the transistor on a grounded metal heatsink by preventing the case from shorting to the heatsink.
What is the circuit supposed to do? What is the coil?