lawn mower blade bolt broke

That is going to require drilling a hole in the bolt and using an eaze-out to remove the remainder of the bolt.

Careful to drill the hole as straight as possible so as to not damage the threads inside the crankshaft.

Spray lubricant should be used and allowed to penatrate for a couple of hours beforehand.

Drilling out is a good way to remove the broken bolt. You could try using a reverse pitch drill bit. Also known as a left handed drill bit. I would drill a centering hole 1/8" and then saturate the broken bolt with some rust penetration penetrating oil (not WD 40) and then use the left handed drill bit, one that is about 3/4 the size of your bolt. This bit must be used in the reverse rotation of your drill. When the bit grabs the bolt, then it will back out the bolt. I have tried eaze-outs and they have broken on me, this method works great. Sometimes you might have to heat the shaft where the bolt is stuck in. Be careful if you use heat, you do not what to damage the engine or cause a fire. Break Out is a good penetrating oil to use. Tapping the side of the shaft while removing the bolt sometimes helps as it loosens the rust. A left handed drill bit can be found here,

http://www.mytoolstore.com/hanson/hanson...