Chainsaw Repair

ok learning slowly about to try to make punch   got a 5/16 grade 8 bolt set up in  lathe will turn down shank to .250  to fit inside press leave shoulder in middle make easy to remove when it breaks cut set screw groove. will have to wait till I get punch block to cut length  on punch end. i need a dimension on a unused 3/8 lp rivet to know proper size for my punch end.  found a 1/2 inch 7 inch long fine thread bolt will make a rivet head former. if someone would give me dimension on rivet head forming dimple at end of  bolt it will help. this may be from any brand that forms a good rivet head. parts I got with it were for regular 3/8 I am making parts for 3/8 low profile