I have been noticing and lower end knock on my 1965 S65. I have a aftermarket crank std stroke. I removed the cylinder head and put my fingers on the piston and rotated the crank. I noticed about halfway down the stroke there was a flat spot where the piston doesnt move but the crank does. FYI the knock was there with the stock crank as well that is why I changed the crank. What can cause this knock? Are the cases worn?