I see , I'm doing the exact with the truck rears I'm making for the 14th scale . I was trying to use the tamiya insides but not sure they will work . I have torn they're pot metal gears apart on my heavy haul tractor . You'll have to look beyond most local hobby shops for the right stuff . I found this place ,
http://www.qtcgears.com to get my id ring gear for my twins . they're close in price as the 1 Ari gave me a link to .