The following is for model 785 296031 065 Sears Roto-Spader:
The belt is 3/8 x 23 in. Gates belt (6723) or equivalent. Make sure it is the GREEN colored belt for it is especially made for machinery (garden type). They cost between $8.00 and $10.00. When installing loosen turnbuckle almost to end of threads. Place belt over spader pulley and idler/tensionor pulley. Replace belt guards and wire guide and tighten belt guard bolts. While holding the motor deck at angle, position belt over back half of motor pulley and gently lower onto deck and slide back to align deckbolt holes and replace/tighten deckbolts. Hook linkage together making sure it is still loose.
Start tiller and engage tines...adjust turnbuckle just enough to engage tines. Avoid over tightening belt tension as it may not fully engage the selector into locked position or shorten belt life. New belts seem to operate tight at first so engagement might occur at first...this will quit after a couple of minutes. Consult Sears Owner manual for further instructions concerning this procedure or seek out qualified professional if unsure.