BQ Transistor Insulating Bushes are utilized primarily to ensure that the fixing bolt of a transistor is separated from the metal tab and body. The reason behind this is to promote maximum insulation of the parts in question. 

If you want to ensure the best possible insulation, it’s recommended that you pay attention to the shank length (also known as dimension ‘D’) of the insulating bush. It should be long enough to locate inside the heat sink bolt-hole. Additionally, it’s important to make sure that the insulating pad fixing hole has the same dimension as the insulating bush shank width (also referred to as dimension ‘B’). 

The maximum operating temperature for these insulating bushes is 125°C, so it’s important to be mindful of this when selecting and utilizing them for your needs.

More information on transistors can be found here.