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So went to mow the lawn and midway the tensioner pulley was off. Does anyone know the size of the nut? Dont want to go to ace and buy a bunch of nuts if i can avoid it. Also wondering how it came loose. Not sure if it is supposed to be a locking nut or just a standard nut.

M10-1.50 Nylon lock, Smooth Flange nut. Yellow Zinc plated. For deck idlers and drive belt idlers.

Either it was too loose to start with or the bearing is seizing and unscrewing it .
Check the pulley carefully

Ty for the help. So it was the bolt that holds the pulley bracket on to the deck.. Turns out it was stripped on the bracket itself so I just added a lock nut underneath it so I'm hoping it holds ok. I mowed the entire lawn with no issues. Fingers crossed