Thai, The motor drive spindle & the gearbox just slide together - it's a fitted shaft. It's probably wear on your large sprocket teeth COMBINED with the bump-stop adjustment on the wheel itself which is causing it to jam. So long as you're careful, if you've detached the window from the crank arm, you can run the thing while you're holding it. That helps to see what's going wrong, but of course, you don't have the weight of the glass, which makes a difference. Sometimes 'adjustments' need to be made with screwdrivers, even hammers! cheers, Graeme