The new plugs you have bought should be ok as modern sparkplugs gaps are pre-set(predominatley)these zetec plugs should also be long life which are normally ok for 60-80k miles,so if not faulty they need not be replaced,if your engine misfires it maybe a plug but more likely to be the coil pack as this is somewhat of a common fault on the older zetec's,hope this is of assistance to you.thanks
Rich,roadside assistance patrol mechanic., May 2011
