Step 1: What You Need
What you need:
A sonic toothbrush (of course!)
A regular toothbrush with some meat around the handle's midsection. (I bought a generic brand twin pack for $2.00!)
A small drill bit for pilot holes. (I used a 1/16 drill bit).
A larger drill bit the approximate size of the brush head shaft. ( I used a 9/64 for my sonic S320).
Step 2: Cut
Secure the toothbrush and cut the handle just below the fattest part of the brush's midsection. I used a hacksaw.
Step 3: Drilling
This is the hardest part because you need to drill a pilot hole as straight and centered as possible. You can use a punch or an awl,nail,etc. to mark the center and make a starting point for your drill bit. Drill a pilot hole and don't be concerned if you drill through the top of the handle.You're basically looking to have a straight, snug hole for the shaft of the sonic when you're done. Find some cheap toothbrushes to use until you've perfected your hole drilling skills and you'll be good to go!
Step 4: Final Drilling
Now you want to use the drill bit that is closest to the size of the shaft on your sonic but it should be smaller rather than larger. If it's just right, you're done! Just slide your new brush head on the shaft and you just saved yourself some major bucks! If it's too tight, (you don't want to use extreme force and possibly damage your brush), put the drill bit back in the hole and continue drilling while applying pressure around the circumference of the hole in order to get it to a size just right for a snug fit. When you've come up with the right drill bit to use, kiss those expensive replacement heads goodbye!