I have 1450 w/44A deck. Pretty sure the configuration is the same for the 1250, 1450 and 1650. I use a 1/2" x 79" Kevlar belt form TSC. I buy 2 of them at end of the season (when they are discounting belts). 2 last me all season. I use my mower for cutting horse pastures. Sure that is the only reason I go through 2 a season. Pretty hard duty on deck and machine. But it's been doing this level of service for 12 years and still going strong. Love this machine....
