I thought they were metric and had a 13mm head or were a 3/8 bolt. I would love to measure mine but I sent them to another member and I can't remember who that was now either. If there's captive nuts, go to your local hardware store and use trial and error in the parking lot. Otherwise, if you have to buy the bolt and nut then make whatever you have (standard or metric) work.
Luck Favours The Prepared