Clean up the pulleys before mounting and run it a short while under a modest load, then expect to retighten the belt. If it starts to slip or smoke cut it off and retighten. This belt works great 6-7 hrs in, but it had to be retightened twice. Belts stretch quickly at first, so I consider this a normal break-in. Just glad I have extra room to tighten again, a few times if necessary, and with sharp bits this old ST-II chips like new.

Lawnmower

This belt worked great. Did not have much stretch to it at all.