the thread pitch is slightly larger than the old pitch if not the same.
Thats your problem slightly anything wont work it has to be exactly same !!!
M12 coarse pitch is 1.75 Fine is 1.5 or 1.25
all we really use over here now is metric and i have never seen a M13 bolt its M12 or M14
1/2 whitworth with is the old British standard had 14 threads per inch and a pitch of 1.814 invest in a pitch gauge this will help and wont cost the earth
are you sure that its not 1/2" with is 12.7000mm m12 will pick up in it and allow for the difference in pitch