Join Date: Nov 1999
Location: Central California, USA
You could drill it and tap for a 1/2" bolt. You need a starting tap and a bottoming tap. They should be high speed steel and of good quality, you don't want to break off a tap in there.
The common vermont american type are not good enough.
If it's a steel crank follow the above.
If it's a cast crank it'l be softer and a cheaper tap might work.
The older I get, the faster I was!