My 66 has factory air, so the water pump belt runs off an idler pulley. The bearing in my original pulley felt pretty notchy. The new one I got had a machined pulley while the stock one was stamped steel. It had a considerably wider groove than the stock one too, almost like it was for a wider belt like the power steering pump uses. I measured the depth of the belt at .300. With the belt seated in the pulley it measured .120 below the edge. I measured the depth of the groove and it was .520. That means I have .100 clearance before the belt could bottom out, so it should be fine. I had to get the special shouldered and keyed bolt out of the old pulley. It looked like the cover was spot welded in two places, but after I drilled them out it wouldn't budge. I decided to just grind the edges until I could pop it loose, not elegant but it worked. I thought I should post this in case anyone else runs into the same problem.
