I have a harborfreight drain cleaner & I want to replace the cable with A 75' cable, will a Ridgid 75' cable fit ?

It should if the diameter matces, not all rooter use the same diameter. This is a power unit right?

Yes , the cable is 50' long I need 75' of cable

As Bob pointed out, It all depends on the cable diameter, Are they the same? Let me know, Tom

Cable diameter is certainly a factor and drum size/capacity is as well, however, the most important factor here will be the cable CONNECTOR ends. If the cable connector ends mate between the harbor freight and the ridgid cables then you will be safe to use the machine to snake the drain. If the ends do not mate up perfectly then you probably cannot use the ridgid cable on the harbor freight machine and there will be no reasonable way to connect the cable to the machine. With the incredible torque created on these connectors you do NOT want to get creative here... could lose the cable-end/cable underground and that will cost you wayyyy more money to get out than a new cable (of course, you could always rent a longer cable machine at a local tool rental store).

Good luck!

Mark