The rear drive belt on my MTD ride on lawn tractor kept coming off. I bought a new OEM belt,replaced it but still have the same problem. Any thoughts. Thank you

Sounds like your belt guides aren't on correct.
Also check your idler pulleys make sure they are turning freely.

Hi,
The bolt(s) that hold the front of the transaxle to the horizontal stay are missing. The transaxle will twist and derail the belt under load. Several styles have been used from a 1/4" bolt to a pair of thread-cutting bolts.
Peace,
Clarke

Thanyou for the help. I have the old version of the 600 Series Transmatic. There do not appear to be any bolts missing but there is only one belt guide bolt,should there be more ? All pulleys rotate freely. How much spring tension should there be on the idler pulley ?
Thank you

Hi,
It was not easy to find a picture with the bolt in it. Most do not have the bolt shown. In the picture it is item# 52. It is 1/4" - 28 x 1.75" with a lockwasher and not above (items 14 and 39) that fasten it. I have generally used Nyloc nuts for extra security and holding ability.
Peace,
Clarke