Verify that the surfaces on the back of the z_tops are smooth and flat.

Verify that the surfaces that will be in contact with the X axis (X ends) are smooth and flat.

As in the previous step, test the 8mm hole by fully inserting a Z smooth rod.

If it is too hard to insert the smooth rods you can use an 8mm drill or reamer to increase the hole diameter. Don't pass through the full depth of the hole, only half of it is generally enough and will ensure you keep a tight fit. Remember, don't use a power tool, use your hands :) !