Will a spark plug for a 1.8L engine fit on a 1.6L engine of the same year Mazda 323? Asking because a friend gave me a set of platinums that were barely used.

 

Check the part numbers, search the web, go to an auto parts store where plugs are sold, or go to a library and get a Haynes or Chilton and look up the part nos. in the equivalency tables. Three factors are critical. plug reach, electrode gap, and plug heat retention (determined by the mass and shape of the ceramic cone supporting the center electrode). Don't just hope for the best or you could damage your engine.