[duke36]

I tried the 18 tooth gear in the stock housing for an overdrive set up for a late '31 with 3.78 rear end. The smaller diameter wouldn't engage the driveshaft gear properly even without the gasket but not sure exactly why.
Check other issues as stated by others. Can the driveshaft gear be installed in the reverse position, i.e., rotate 180 deg? Some other posts may have touched on this