Crank slipping

Hi there!

Thanks for reaching out to us. We're happy to look into this. Can you show us a bird's eye view of the operator's arms that move the sash? That will help us confirm if we have a comparable match. 

Thank you for the photos! The operator shown in your photos reminds me of the 39-253RB and 39-439B. The important difference between the two is the guide's design which is attached at the end of the arm. The video below will show you what I mean. Take a look and see which one matches up with your old assembly. 

Please note that the 39-452 is sold separately.