Toro self propelled mower rear wheel keeps falling off

Are these regular bolt on wheels? Have you changed the nut?

I would expectc to find a "Stover" nut on such an appllication. Stover nuts are similar to "NY-Lock" but without the nylon lock. Rather, Stover (and you have to ask for them at the hardware or auto parts place), are not round and/or have had a certain side portion punched to make the nut hard to put on. Some well meaning back yard mechanics (read me) have 'fixed' the Stover by running a tap through them maning them easier to get onto the threads ... but alas, it didnt stay. That was how I learned about Stover.

Let me know if this helps.

A