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I replaced engine on a 1999 Yard Machine tiller and now the transmission belt doesn't have enough slack to allow it to be moved from neutral to a drive/tine gear even with the idler/tension pulley not installed. I've moved engine to allow as much slack as possible. It calls for 1/2 x 58 in. belt. Would increasing belt size remedy and if so, how is it determined what size to get?

I would say you put the wrong pulley back on. Maybe from the new to you engine?

I used pulley off of original engine

To measure, what you need take is a 1/2 nylon rope and feed it through all the pulleys the way the belt goes. Mark the length and then measure it.