Haven't  got it  ?  Need a  special  size  i.e. WHITWORTH ....Then  > MAKE  YOUR  OWN ...

FIND A NUT OR BOLT of the size  you need ...One from out of the motor is  best i.e.  Crank or Head  etc , because they are hardened more than regular  bolts .  Split the nut with an angle grinder  [ DO NOT CUT IN HALF  ]  .   Now  with a  good  file or DREMEL  > make 3 grooves [  1 at the split  ]  equally spaced  in your nut or on the  bolt thread .  These will remove  metal chips .  Now  hold  the piece that needs threading  in a vice .  put the NUT or BOLT in/on it .The split will allow  the nut to flex/ open  & the bolt can be tapered on a  grinder ... Grab NUT or BOLT  with a VICE -GRIP  &  rotate  this   will cut the  thread .  Repeat  process [ tighten  VICE - GRIP  ]  until you have cut to the desired depth [ Keep trying an unmolested NUT or BOLT until you have desired fit  ]   ... , Lubricate with oil as you are cutting &  use  light pressure with vice grip for final cut  , to get a nice clean  thread  ...PRACTICE  MAKES  PERFECT ...