Why are the mower blades hitting each other while engaged but not when disengaged. The mower is a cub cadet RZT 50.

If that is the case it sounds like bad bearings. When not moving they will hang to the center of gravity. When engauged they will be pulled to the torque of the belt. Are they loose?

What length blades did you purchase? It looks to be 17.9 for blade length.

When you engage blades they flatten out there needs to be at least a 1/4 inch clearence all around