As you can see from the photo below, the key for my expensive bicycle lock snapped in two from many years of use. So I repaired the key, as shown below.

The broken key.

The bicycle lock.

Repairing it was simply a matter of finding a way of holding my key firmly somehow and drilling two holes in the top part of the key's cylinder, as far from the open end as possible. To hold it I put it in my table vice, but C-clamps or any creative trick you can come up with will do. Make sure you use a drill bit that's made for drilling metal. However you hold it in place will likely distort the roundness of the cylinder which will make it no longer work in the lock. To make it round again simply squeeze it back into shape using pliers or by putting it back in the vice or C-clamp and compressing it until it's round again.

Drilling the holes. Note that this is a small vice bought at a hardware store and clamped to the table with two small C-clamps.

Making the hole round again. Hold the pliers near the ends of the handles for maximum strength.

As you can see above, I put a smaller key ring through the holes in place of the flat piece of metal that broke off.

And from the photos below you can see it's a simple matter to stick the key in the hole and, while gripping the smaller key ring between my fingers, turning the key to lock/unlock the lock.

Voila! It took less time to repair it then it did to go out and buy a new one and it's good for another 20 years.

And if the lock ever becomes difficult to turn, spray some WD-40 inside the two large holes in the lock. That knocks any dirt/rusty bits loose. I usually then squirt in some bicycle oil to grease it up a bit. After applying the WD-40 and/or oil, use the key to turn the pieces inside the lock back and forth a few times to spread the liquids. WD-40 can be found in any hardware store and bicycle oil can be found wherever bicycle supplies are sold.

WD-40 and bicycle oil.

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