On the 1991 nissan maxima the distributor cap is removed with three screws. The rotor is supposedly held on with one set screw. I remove the set screw but the rotor will not come off even by prying up with screwdriver. I thought it was corroded on and tried rotating it with a slip joint pliers. I got about one sixth of a rotation on it but no upward movement. I replaced the set screw and tried to start the engine. It was completely out of timing. I removed the screw and looked in the hole;
there appears to be nothing at the bottom of the hole to seat the screw on. The rotor now can be rotated the one sixth rotation even with the screw tight therefore I cannot control the timing. What is the secret here? The rotor should just slip off but it will not. Could the end of the shaft be broken?

 

On these cars there are 3 screw that hold the distributor cap on, followed by a 8mm set bolt holding the rotor. As long as you did not remove the entire distibutor "you might be able to salavage the timing". How much forced did you use while trying to remove the rotor? There is some play with a higher mileage engine.

 

are you sure the set screw didn't break inside there?
there should be very little to zero corrosion in there. the cap has a gasket that seals out water and etc. the other thing i can think of is the screw/bolt is broken part way in the distributor hence preventing you from pulling up on the rotor. without the screw the rotor should come up VERY easy.