AVB, I've given up buying belts for my self-propelled mowers. They stretch in no time these days and fly off. Forever re-fitting and replacing them. Easier to just push the mower like we did in the old days.
You maybe installing Poly belts and need to switch over to Kevlar belts. Poly do have a rather large stretch factor of 3-5% where Kevlar normally only stretch 1-2%. Sometime though you have no choice but to buy OEM belts due to sizing specs. Many OEM intentionally use specs that only they have especially OEMs like MTD. Many of the so-called aftermarket belt for MTD simply will not work properly.
