I've mounted transistors to heatsinks before and used off the shelf kits (both for the heatsinks themselves and insulation), but I'm now creating a custom solution for a project, based on a single piece of aluminum that I'm cutting and drilling (and need to mount four transistors to).
This got me thinking: the plastic insulator I'm using (alongside the mica pad, to ensure the bolt and the transistor don't short) isn't as deep as the aluminum. The bolt should therefore still be insulated, but there will be some part that *isn't* shielded by the plastic internally, with an air-gap between it and the metal.
This is probably 'normal', based on some Googling, but should I be looking for a deeper plastic insulator instead that lies flush either side?