hi faisal firstly u have to specify the gear either spur or helical. eventhough both be gears backlash is measured by the same technique but the difference lies in acceptable backlash. for measuring the backlash u have to keep an equal working surface of both pinion and girth gear. then u can take a feelers gauge and keep the guage in the non working surface at the intermediate place not at the addendum or dedendum.acceptable backlash may be about 10 mm. the new girth gear should be having about 3.8 mm. this backlash varies from kiln to mills.