Sounds like the crank is tapped for a 1/2-20 bolt. Get one at the hardware store and try it. If it works, great. If not, drill with a 27/64 bit and tap with a 1/2-20 tap.
Pulling a damper on with a bolt is risky business. It is very easy to break the bolt in the hole. Be sure to lube the threads and crank snout with anti-sieze. You will probably also have to get a bolt that is an inch or two longer to start the damper on with. A damper installation tool is best. Here is one example.
http://www.lislecorp.com/tool_detail.cfm?detail=959