2020 Mercury 250 Pro XS ser. #2B743128
For some time now I have been trying to eliminate my prop nut from backing off the prop shaft. I contacted Mercury and the associate was very helpful. However he personally had not heard of or seen any bulletins addressing this. His first suggestion was to replace the nut and washers which I told him I had done but the results was the same. He did find some notes where before torquing the nut to tap prop with a plastic mallet to ensure the prop was seated fully on shaft. So I removed the prop and the hub. I thoroughly cleaned everything, from the edge of the seals to the end of prop shaft, including the washers and nut threads. I then applied Mercury prop grease to the splines as outlined in the manual, taped prop with plastic mallet to ensure it was seated, assembled washers in the correct order and torqued nut to 55 ft. lbs. I marked the nut and end of shaft with a sharpie in a straight line. I went to lake, launch my boat, tied it to the dock about 10 feet away. I parked my truck went back to my boat and got a phone call with an emergency at home. I put my boat back on the trailer and of course I had to use the motor to get it to the bow eye. The motor rpms probably went up to around 2500 or so. The picture below shows exactly how much the nut moved after I got it on the trailer. There has to be something going on and if I don't correct this the nut will continue to back off the shaft. I have attached a picture of the nut and end of prop shaft showing how much the nut moved after putting the boat back on trailer one time.
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