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lower unit diagnosis

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  • lower unit diagnosis

    I picked up my new old and pitted lower unit today and while I was there we figured out what happened. I told him about my ex-mechanic problems and also how the water pump on the motor was new. He gave a look like "are you sure?". I know I paid to have it replaced this fall so he took it apart and after doing so, the driveshaft litterally fell on the floor. We looked at the impeller and he said it looked like it was at least two years old which made sense because I had the entire water pump housing and impeller replace after I bought the boat. Looks pretty evident that my ex-mechanic wasn't doing crap.

    The kicker was the driveshaft. It broke off right at the base of the impeller. I expected that it would look like someone had cut through it with a hacksaw, but only about 15 - 20% of the actual surface area looked like it was sheared off. The rest of it was rusted so I'm pretty sure that it was just a matter of time before it sheared off and thank goodness it didn't happen when I was 20 miles offshore. I didn't get a picture, but I'm still amazed that the drive shaft lasted as long as it did....and I am even more curious as to what caused it to start to fail.
    1978 Mako 25 - Blind Hog
    1985 Mako 20c - sold
    Fort Walton Beach, FL

  • #2
    In this day and age if you get an honest mechanic your very lucky, I have heard horror stories for years of the way the boat mechanics rip off there customers. Sometimes there just so busy they dont really know what they have done and not done. Thank goodness there are still a few good ones out there. Sailor there have been a few of those lowers on e-bay lately for a reasonable price. I wonder if its just time for your old 150 to go away. hope it turns out well . As to what caused it to fail, years of saltwater and not enough flushing after each trip would be my guess.

    [br]1994 Mako 215 Dual console Optimax 225[br]1978 Mako 19 with 90hp johnson[br]1996 Mako 22[br]1982 Mako 171 Angler 135 Black Max Mercury[br]1987 21b 225 Yamaha[br]1974 23 inboard Gusto gone.[br]1979m21 225johnson \"blue dolphin\" bought off this board and restored [br]with everyone\'s help!!Gone but not Forgotten....[br]1979 20 Mako 115 Suzuki gone[br]1977 19 Mako 115 Johnson gone[br]1976 23 Mako twin 140 Johnsons gone[br]1983 224 with closed transom and bracket[br]And 162 SOB (some other boats)[br]Venice Florida, Traverse city Mi.


    • #3
      Dave's right.

      I had to hacksaw my impeller off - it was rusted to the shaft..wouldn't be surprised to find rust ANYwhere in the L/U. I religiously flush my motor, trailer, underbody of the van, etc, every use. (Religiously = "Please God, don't let my drive shaft break 20 miles out".

      \'73 22 Mako Reedville, VA


      • #4
        The type of wear pattern you are describing is typical of a shaft that is not aligined correctly and bascially has a bow in the shaft between 2 bearings (mounting areas). It's called a fatigue fracture and all shafting do eventually break from this phononima. What happens is the shaft is stressed (put in tension and then in compression) on one side and then the other from the "wobble". Eventually the shaft starts to fagitue and a small crack starts at the spot where the highest stress' accure. It keeps fracturing (cracking) a small amount each time the shaft is rotated a full turn until the shaft can no longer transmit the torque (power) that the motor is putting out - to the prop in this case. The reason that you only see a fracture on the shaft of about 20% is that after the original crack started, each revolution is now polishing (grinding it smooth) the cracked area so when the shaft does completely fracture you see that typical 20% that looks broken and the rest is very smooth. Now, my question would be who was the last person to remove and reinstall the lower unit?? Was any of the gears of the lower unit removed (for work) and then reassemble?? That would/could account for the out of alignment of the shaft between the 2 (upper and lower) bearings or in this case the motor's output shaft and the lower unit. Or one last thought was that the shaft was bent to start with (from the factory??) and or you hit something hard enough to bend it slightly to cause the wobble??? Just some things to think about.



        1973 22 CC Milford, CT USA[br]