Roller Chain, Tools & Chain Guards

How do I identify what roller chain I have or need?

• 

• Chain Dimensions

  Chain Number	Pitch	Between Inner Plates
  25	1/4"	1/8"
  35	3/8"	3/16"
  40	1/2"	5/16"
  41	1/2"	1/4"
  50	5/8"	3/8"
  420	1/2"	1/4"
  428	1/2"	5/16"
  520	5/8"	1/4"
  530	5/8"	3/8"