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Alpha One Gen One lower unit rebuild

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  • Alpha One Gen One lower unit rebuild

    I have twin 4.3 Alpha 1's on my 88 Chris Craft 290. I pulled the drives off this winter and went through both of them. One drive has no shims behind the carrier. Is that common? Because all gears, bearings and races looked good I just replaced oil seals, o rings and gaskets. I have a mercruiser service manual #6 . I have all special tools except except preload tool to check backlash.Do I have to check backlash before putting these drives back in service? Both drives ran perfect all summer. The only reason I tore them down is because they are old. Lastly when installing the carrier on the other drive with shims, which one goes in first, the black split one or the others. Thank you in advance for any feedback. This is my first time in my life I have ever posted a question on the internet.

  • #2
    Re: Alpha One Gen One lower unit rebuild

    Most drives do have shimming of some sort. It's not unheard of to come across a drive with no shims in it, it's just not common. For a drive to have no shims in it, that would have to mean the machining of the drives is perfect in every dimension.

    Now what the backlash is, is a way of determining wether or not the gears are properly spaced apart. (and the spacing is what the shims are for). Without checking it, you have no way of knowing if it is correct or not. And just because no shims came out of one drive, doesn't necessarily mean it's correct either. If the guy before you forgot to put them back in, then your not going to find any shims in there when you pull it apart. On another note, if your drives are counter rotating, the counter rotators gears are going to be reversed.
    just because you found it that way does not mean it is supposed to be that way.

    Comment


    • #3
      Re: Alpha One Gen One lower unit rebuild

      Hello and to iboats.

      If you haven't changed any of the gears, bearings or housings, then just reassemble as it came apart. No need to re-shim.

      As for the shims behind the carrier, the parts drawing only shows a single 'thrust ring', no shim are shown in this area, and it's not a split ring, however, the manual shows a shim (let me do some more research).... The only black split ring I know of in the drive should be in the top housing behind the main input bearings. Refer to the manual, page 3A-29, step 17...

      Chris......

      BTW, congrats and well done on your first ever internet question...
      Last edited by achris; March 4th, 2014, 01:56 PM.
      xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
      The world takes on a whole new perspective when viewed from 100’ below.
      1972 Bertram ‘Bahia Mar’ 20
      2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
      (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


      Comment


      • #4
        Re: Alpha One Gen One lower unit rebuild

        Originally posted by achris View Post
        ... (let me do some more research)....
        Have a look at this parts breakdown...

        http://www.mercruiserparts.com/Show_...+SHAFT%29+-+MR

        No shims shown to adjust reverse gear backlash... Interesting, because there should be.. Have a look at the previous model (the 'R' drive), the same apart from the water pump housing and the way the lower drive shaft is handled...

        http://www.mercruiserparts.com/Show_...ller+Shaft+-+R

        See part #72, that's the shim/s...

        I suspect it's just an oversight on the MR/Alpha One drawing.

        Chris....
        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
        The world takes on a whole new perspective when viewed from 100’ below.
        1972 Bertram ‘Bahia Mar’ 20
        2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
        (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


        Comment


        • #5
          Re: Alpha One Gen One lower unit rebuild

          Chris in the lower, on a standard rotation, the forward gear shims are located behind forward gears bearing race. This moves forward gear forward and back as needed. And the reverse gear shim is located between the carrier and a lip in the lower unit that carrier sits up against when it's torqued down. This moves reverse gear forward and back.

          And then of course the pinions shim is under the race for the driveshafts tapered roller bearing.
          Last edited by JustJason; March 4th, 2014, 02:01 PM.
          just because you found it that way does not mean it is supposed to be that way.

          Comment


          • #6
            Re: Alpha One Gen One lower unit rebuild

            Originally posted by JustJason View Post
            Chris in the lower, on a standard rotation, the forward gear shims are located behind forward gears bearing race. This moves forward gear forward and back as needed. And the reverse gear shim is located between the carrier and a lip in the lower unit that carrier sits up against when it's torqued down. This moves forward gear forward and back.

            And then of course the pinions shim is under the race for the driveshafts tapered roller bearing.
            Hi Jason.

            ^^^ I think you mean reverse gear...

            I am familiar with which shims do what (having rebuilt several dozen of these drives ), I'm just commenting that the shims for reverse gear are missing from the parts drawing for the MR/Alpha One ....

            Chris......
            Last edited by achris; March 4th, 2014, 02:02 PM.
            xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
            The world takes on a whole new perspective when viewed from 100’ below.
            1972 Bertram ‘Bahia Mar’ 20
            2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
            (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


            Comment


            • #7
              Re: Alpha One Gen One lower unit rebuild

              Originally posted by achris

              ^^^ I think you mean reverse gear...
              Lol, yup... I double forwarded myself here. Thanks. I went back and fixed it.

              But part of what I was saying is because of what you wrote earlier, when you wrote this;
              Originally posted by achris
              As for the shims behind the carrier, the parts drawing only shows a single 'thrust ring', no shim are shown in this area, and it's not a split ring, however, the manual shows a shim (let me do some more research)....
              They are there regardless of what the parts drawing shows. All the drives up to the gen 2 shim all 3 gears. Gen 2's only shim pinion and forward.
              just because you found it that way does not mean it is supposed to be that way.

              Comment


              • #8
                Re: Alpha One Gen One lower unit rebuild

                Thanks for the feedback guys. Because both drives ran flawless all summer I think I'm going to put it back together without any shims on the drive that had none. (Both drives are right rotation) The drive that had shims behind the carrier is where I am now. There are four shims , one is a black split ring. Do I put that split ring in first or last against the carrier? Thanks again guys for your help. This is a great site.

                Comment


                • #9
                  Re: Alpha One Gen One lower unit rebuild

                  IF it's the lower unit we are discussing, the shims go in first... If it's the top box, then the black split ring goes in first... You have the manual, it's in the exploded drawing on page 3B-10, the shims are part #17, the ring is part #19....
                  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                  The world takes on a whole new perspective when viewed from 100’ below.
                  1972 Bertram ‘Bahia Mar’ 20
                  2006 Mercruiser 4.3MPI (0W617679) w/Alpha One Gen II (0W829301)
                  (Original - 1972 '165' In-line 6. Previous - 1994 4.3LX)


                  Comment


                  • #10
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                    Re: Alpha One Gen One lower unit rebuild

                    Thanks for the reply. #19 is the thrust ring (attached to carrier) #17 is a shim pack for my drive. There are four shims one of them is a black split ring. Not sure if it matters if it goes in first or last.

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