speedo drive, too small?!?!?!!? on '77 kz750

so i'm finishing up my bike, i bought a speedometer drive gear, and it's like 1mm to small in diameter to fit on my axle, so i bought another one off ebay, figuring it won't happen again, but it is, is there somthing i'm just not understanding, i can't find scenario to make it work, please advise me, thanks mucho

If you don't have the part number you can get it from the Kawasaki web site. Make sure all the parts that it needs to fit are in fact the right parts. Is the axle the right one for the bike? Someone before you could have changed the front end.