Take the mower down to the local hardware store and get the appropriate bolt and heavy duty washer (one of those real thick ones) to secure the pulleys. There should be a threaded hole in the end of the crank for this. Also a couple of things before you run the bolt up. Did you put anti-seize compound on the crank before you put the pulleys on? If not it's a very good idea. You did install a shear pin didn't you? And you do not need to use a lockwasher on the crank bolt, put some Locktite on the threads instead.