Most likely a 1/4-20. Any 1/4" bolt will work, as long as a nut of the same thread pitch is used. An Allen bolt makes it a lot easier to adjust the timing.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!