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Use 5/8 closed end wrench on tensioner pulley bolt then lift tensioner up and away from belt to relieve tension. Here are instructions, let me know if you have questions. Thanks.
Make sure your not trying to put the belt on when you put the tensioner on.....You can try to tilt the tensioner upward in the released position to see if that helps...Also make sure bolt goes into hole without tensioner to make sure its not a thread problem......If still having issues I'd check the old tensioner to make sure the part is the same. Let me know if you have questions, thanks.