What could cause a Merc Lower unit to Grow?

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  • May 11, 2015 at 4:49 am #1462
    benchrascal
    Participant

      Bought a Merc 4.5hp today and the seller told me it was good to go ,just needed a shim (extension) in between the housing and lower unit .At this point in time I’ve heard so many ridiculous stories about what should and shouldn’t be done to outboards that i just stayed quiet and paid him the $25 i thought the motor was worth .Poking into it tonight there was no way the lower unit would fit completely and have both of the nuts do up satisfactorily .Grabbed another complete lower unit from my stash and sure enough it popped right in.I measured both and they seemed to be identical and now the only thing I can think of is the original driveshaft must have a slight bend in it which would cause it to bind .Never come across anything like this before ,anybody ever experienced something like this?

      May 11, 2015 at 5:01 am #15698
      legendre
      Participant

        I have one of these motors, a 1983-era example – it has the CDI ignition, for what that’s worth.

        So I take it that your replacement lower (the one which +did+ fit) had a driveshaft in it, already? If so, what’s the geometric difference? Which measurements don’t agree? It’s certainly easy enough to check the driveshaft(s) for warp – set them side-by-side – or roll them over a surface plate.

        Look for other differences that could prevent assembly, like splines cut too shallow and so forth. What’s stopping it?

        May 11, 2015 at 5:07 am #15699
        jeff-register
        Participant

          US Member - 2 Years

          I have heard of auto trans parts for a Chevy be different from the others as they were made in a different country for Chevy that year. Why not boat motors? Ever look at a new Cadillac paint job? Orange peal plus terrable paint job. Replaced a windshield wiper motor from GM & the new one had wires not soldered to the terminals yet, NO reversing or stop connections yet!

          May 11, 2015 at 9:44 am #15703
          billw
          Participant

            US Member

            Sometimes the shift shaft lengths are different, too; so don’t overlook that…..Like Legendre says, there are splines cut at different lengths, depending on the year, as well as the overall length of the drive shaft, that may be different. Mecrurys are so fickle about swapping parts that I rarely even try. I remember when you couldn’t interchange big lower units because they moved the side stud location by only an eighth inch. Was that really necessary? Who knows. Not me.

            Long live American manufacturing!

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