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mercury lower unit shifting

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  • mercury lower unit shifting

    so i just bought a lower unit off of ebay for my 67 Merc 500. my original lower unit had so much corrosion it was breaking apart and the bearing carrier was stuck in the case.
    I'm not familiar with the shifting mechanism on these mercury outboards, First mercury outboard I've owned. so on both lower units the prop spins in forward when i turn the drive shaft engine rotation (Clockwise). when i turn the shift shaft on the original lower unit it spins almost 360 degrees (Probably 315 degrees) and i felt it contact nothing except for the sides of the case i presume. the lower unit i bought off of ebay (Freshwater unit & very clean looks like no one ever opened it up) the shift shaft spins 360 degrees and continuously spins in either direction and i feel it nothing as well. i know there is a cam follower pin in the end of the propshaft that rides on the shift cam that shifts the clutch dog.

    Anyone have input on these units and how they shift when not connected to the motor?

  • #2
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    The rear of the F gear is a sawtooth design. The front of R gear uses square cogs. The shifting fork moves the clutch dog in both directions to engage one or the other. The shifter cam, connected to the shifter lever, has 3 detent positions, shallow, mid depth, and full depth. As you said the depth of the groove in the cam determines the position of the "cam follower" on the prop shaft and the pressure on the spring controlling the clutch dog.

    In F gear you have the least pressure and the dog engages F gear. Since upon decelerating, the propeller, driven by the oncoming water at whatever speed the boat is traveling, spins faster than the engine. Were it not for the clutching effect of the sawtooth, the prop would act as a brake and you might wind up with your teeth in your windshield. However it ratchets, allowing different rpms of both driver and driven.

    As a result, rotating the prop, as viewed from the rear CW (engine off in F gear) will cause the shaft to move in and out slightly as the dog slides over the sawtooth rear of F gear. Turning the prop CCW will lock up and if you do cause the prop shaft to rotate, the drive shaft will rotate also and if connected to the crankshaft (in normal operation configuration) you will turn the crankshaft with a good deal of effort.

    Reverse is not considered to be the same situation so the dog is just squared off cogs and slots. If the engine isn't running you have to carefully put pressure on the shifting levers in the R direction while rotating the prop by hand. Once cogs and slots line up the shifter will easily move on into full R position and lock the prop shaft to the drive shaft in both directions.
    If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.