The Joint Electron Devices Engineering Council, or JEDEC, created many standard designs for transistor cases, including the TO-3. Larger than other transistor packages, the metal TO-3 case handles greater amounts of power and heat, up to 300 watts. TO-3 type transistors may mount to circuit boards, but more often to metal chassis panels, as the chassis serves as a conductor of heat. Unlike most kinds of transistors, it takes extra hardware to install a TO-3 type. Screws and nuts hold the transistor securely to the chassis and thermal grease improves heat transfer, keeping the device cool.
Locate the TO-3 transistor on the circuit schematic. An important part of installing the transistor is identifying whether you need to electrically insulate the transistor from the equipment chassis. The schematic or equipment instructions will indicate this.
Put a small dab of thermal grease on the bottom of the transistor and spread it so it covers the bottom surface of the TO-3 case. Avoid getting the grease on the transistor's two straight pins.
Slip the mica insulator through the transistor's pins and onto the bottom, if the schematic indicates it needs electrical insulation. The insulator matches the shape of the bottom surface of the transistor, including the mounting holes. Put a dab of thermal grease on the bottom of the mica insulator and spread it as before.
Fit the transistor to its location on the equipment chassis. The two mounting holes on the transistor's case should line up perfectly with holes in the chassis.
Set the machine screws into the TO-3 mounting holes, so the screw heads are flush with the top surface of the TO-3's flange.
Slip the shoulder washers onto the screws that protrude into the equipment case, if the transistor needs electrical insulation.
Fit the #6 washers onto the screw threads. Slip the ring crimp connector over one screw. Thread the #6 nuts onto the screws and tighten with a nut driver.
Match three wires from the circuit that connect to this transistor. For example, the collector connection goes to the crimp ring. The base and emitter pins are closer to one mounting hole than the other. From the bottom of the case, the base will be to the left or under the emitter pin, depending on its orientation. Crimp the collector wire with the crimping tool. Solder all three connections.