Blew a new mower belt that I just put on September. Only mowed a few times last fall and on the third time this spring it blew apart. It seemed like there was a little extra movement in the belt while it was running, but the tensioner was tight and spring was still strong. While doing some more investigating, I found the center belt pulley on the gear case was loose on the shaft. A lot of wiggle in it. I took it all apart and found that the key was very disfigured and misshapen and this is what the key looked like. I guess over 20 years it was going to wear. At least I will get away with a just a pulley and key as opposed to the whole gearbox, shaft and bearing assembly.