Wondering if someone can help with this one......my motor shaft is 5 mm according to the maufacturer, the prop shaft is 3/16. I ordered universal joints with 5mm on one end and 3/16 on the other end, but they don't fit. It seems as though they are very close, but it is too tight. My concern is the motor shaft, can I take some sand paper and sand it down a bit, or should I just pound it on with a hammer? Any suggestions?

Hello, I would only sand the shaft as a last resort. The metal filings stick to the motor and are extreamly hard to remove. I would get a #9 drill bit (0.1960inch) which is very close to 5mm (0.19685inch) and open the u-joint. You should be able to get the drill bit at any good hardware store or McMaster Carr or Grainger.