If not what echoman says (I agree- check that first), the gearbox isn't hard to rebuild on a 10515 as I did mine this past summer. There are shift keys/ pins in there as I replaced those and both the input/output beveled gears that mesh together when self propelling. Your various gears outside of the ones I mentioned are probably good as they are hard parts. It's not tough if you have to go that route (plan for 2-3 hrs total if you work slow) and parts are available. I think my parts were around $50 or so..... Hope it's an easy fix for you!