How to TO-220 heatsink bolt?

I see many beginners try to bolt a TO-220 device onto a heatsink and worry about alignment, pressure, and electrical safety.

You can bolt a TO-220 device to a heatsink by matching the right screw size, adding an insulating pad when needed, setting light torque, and checking that the metal tab does not short to the heatsink. This keeps the device cool and safe to use.

I want to explain each step in a clear way so you can handle TO-220 mounting without stress and keep your power parts stable.

What bolt sizes fit TO-220 packages?

I remember my first time mounting a voltage regulator. I held the screw in one hand and the MOSFET in the other, and nothing lined up as I expected.

Most TO-220 packages fit M3, #4-40, or M2.5 bolts. These sizes match the hole on the metal tab and fit well through standard heatsink slots.

I learned early that the hole on a TO-220 tab is small but not fragile. It accepts several bolt types, but some fit more cleanly. When I used a larger screw once, I scratched the tab edge and felt worried that I damaged the part. After that, I switched to M3 as my default choice.

Common bolt sizes for TO-220

Bolt Type	Fit Quality	Notes
M3	Best	Most heatsinks support it
#4-40	Good	Common in US kits
M2.5	Fair	Works on thinner brackets

Why bolt size matters

A bolt that is too wide may push the device off-center. A bolt that is too short may not reach the thread on the heatsink. Both problems reduce the pressure between the device and the metal surface. This weak contact raises the temperature.

How I test bolt fit

I place the bolt through the tab before mounting. If it moves freely without scraping, the size is fine. I also check the length. A bolt that is too long may press against the PCB or hit the bottom of the heatsink.

Small tip that helps

If you feel resistance before the bolt reaches the heatsink thread, stop and check alignment again. Forcing the bolt can bend the TO-220 tab.

Why use insulating pads on regulators?

I once built a linear power supply and bolted the regulator directly onto the heatsink. As soon as I powered it up, the output shorted. That day taught me the role of insulation.

Insulating pads stop the metal tab of a TO-220 device from touching the heatsink electrically. Many regulators and MOSFETs have a live tab, so an insulating pad keeps the circuit safe while still allowing heat to flow.

This detail feels small, but it saves many boards. I saw users on forums ask why their supply failed after using a shared heatsink. The answer was almost always the same: the device tab was live and touched the frame.

Why tabs must stay isolated

Many TO-220 parts connect the metal tab to:

• Drain of a MOSFET
  
• Collector of a transistor
  
• Output of a regulator
  
• High-voltage node in power supplies
  

This means the heatsink becomes live if we mount the device bare. An insulating pad stops that.

Types of insulating pads

Pad Material	Thermal Feel	Electrical Safety
Silicone pad	Soft	Very safe
Mica sheet	Firm	Very safe
Polyimide	Thin	Safe
Ceramic pad	Hard	Very safe

How pads affect heat

Pads add a thin barrier between the device and the heatsink. This slows heat transfer a little. A thin pad moves heat faster than a thick one. I always try to use the thinnest pad that still gives strong electrical isolation.

My simple rule

If the device tab is connected to a high-voltage or high-current node, I always use a pad. If I mount several devices onto one heatsink, I always use pads for all of them.

How does torque affect thermal transfer?

I made the mistake of overtightening bolts many times. Once I cracked a MOSFET tab and learned that pressure matters just as much as bolt size.

Torque controls how tightly the TO-220 device presses against the heatsink. Light, even torque creates good surface contact, which helps heat spread faster and keeps the chip cooler. Too much torque can bend or damage the device.

When I tighten a bolt, I watch the tab very closely. If it starts pulling sideways, I stop at once. Tightening should feel smooth and gentle.

Why torque matters

Heat leaves the chip through the metal tab. The more surface area that touches the heatsink, the better the heat flow. When the bolt is loose, only a small area touches. When the bolt is tight enough, the whole back of the tab touches the heatsink.

Signs of correct torque

• The pad stays flat
  
• The tab does not bend
  
• The bolt stops turning naturally
  
• The device does not slide
  

What happens with too much torque

• The insulating pad tears
  
• The tab bends
  
• The device tilts and reduces contact
  
• The bolt strips the heatsink thread
  

What happens with too little torque

• The device lifts on one side
  
• The pad forms air pockets
  
• The chip overheats
  
• The heatsink does not remove heat well
  

A simple method I use

I tighten the bolt until it stops turning freely, then give it one small turn. This holds everything firmly without damage.

Can washers prevent electrical contact?

I used to think washers were only for tightening, but then I found out they can save a circuit when used the right way.

Washers help prevent electrical contact by keeping the bolt head from touching the TO-220 tab directly. Plastic, nylon, or fiber washers create a safe barrier that blocks unwanted current paths.

I discovered this when I mounted three MOSFETs on a single heatsink. One device shorted because the metal bolt touched the tab. After that, I used nylon washers every time I needed both electrical safety and clean alignment.

Why washers matter

A metal bolt head often sits close to the tab. Even with a pad between the tab and heatsink, the bolt head can still make electrical contact. This creates a hidden short that is hard to find. A simple washer blocks this path.

Types of washers for TO-220

• Nylon washers
  
• Fiber washers
  
• Plastic washers
  
• Shoulder washers
  

What shoulder washers do

A shoulder washer slides into the tab hole and keeps the bolt from touching the metal inside the hole. This gives full isolation between bolt and device.

When washers are necessary

Washers are essential when:

• The heatsink mounts many devices
  
• The device tab carries high voltage
  
• You want to share one heatsink between circuits
  
• You use a metal bolt through a live tab
  

How I check for contact

After tightening, I place my multimeter between bolt head and device tab. If the meter shows no continuity, the washer is doing its job.

Conclusion

Bolting a TO-220 device to a heatsink is simple when you choose the right bolt, use proper insulation, set gentle torque, and add washers when you need electrical safety. With these steps, the device stays cool, safe, and stable.