when Greywolf said 30mm, I knew I needed two bolts, not just one for the missing one. The one that was left in there does not come all the way through to the other side. No way could it be 30 mm. So I picked up two at the hardware store that are 30mm. A few of the threads now hang out the other side -- which could be good if they get a bit dirty which would help resist backing out again. Or could put a nut on it as a keeper. The one bolt that was still in there was removed and replaced. It was only 20mm. Thanks again!