Technical Inline 8 263 Head Freeze Plug Size?

I have a 1953 buick special an ive been looking for threads on the sizes of freeze plugs and ive had no luck.
I was able to put get all the freeze plugs for the side of the motor but not the head. Worst part is i lost my measuring tool. Help??? All i need is the head plug

 

Kanter, old Dorman's cat, get another mic.
Press a piece of paper into the hole and get a few that are really close.
I've been stuck for a $1.00 part too, sucks most. Best-o-luck to ya.

 

Thank you I actually got lucky enough to find the correct ones took me a while but I found a shop that carried all sorts

 

8 years is a long time