Bolt size for Monark stem/handle bar bracket

[NewOrleansFlyer]

The old bolt head was getting a bit rounded so I took it to the local hardware store. The best they could do was 10mm...close. When I got home I put it into the stem to see if I could it tighten down. No go...anybody know what size this thing is??? Thanks

 

[5speed]

have them put a nut on your old bolt,itd prolly 5/16 or 3/8 fine thread,monark didnt have metric