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Changing omc upper gear ratio?

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  • Changing omc upper gear ratio?

    Hello,

    I have a 1989 omc cobra..5 liter ford. Upper gear ratio is 21/17 in the outdrive. One of the gears chipped off...causing lots of problems. The case is now gauged around the bearing...and I will need a new case and gears. Question: I have a buddy with an old 1988 omc cobra drive from a chevy 305. The gear ratio is 21/17. If I swap the upper units...how will the change in ratio effect my boat? Or..if anyone has an upper unit to trade or sell.

    thanks..Jim

  • #2
    Re: Changing omc upper gear ratio?

    Did you miss-type ? 21/17 is 21/17

    DHP

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    • #3
      Re: Changing omc upper gear ratio?

      Originally posted by lumpy View Post
      Hello,

      I have a 1989 omc cobra..5 liter ford. Upper gear ratio is 21/17 in the outdrive. One of the gears chipped off...causing lots of problems. The case is now gauged around the bearing...and I will need a new case and gears. Question: I have a buddy with an old 1988 omc cobra drive from a chevy 305. The gear ratio is 21/17. If I swap the upper units...how will the change in ratio effect my boat? Or..if anyone has an upper unit to trade or sell.

      thanks..Jim
      Sorry..mistype..I had 21/18...and switching to 21/17

      Comment


      • #4
        Re: Changing omc upper gear ratio?

        Originally posted by DHPMARINE View Post
        Did you miss-type ? 21/17 is 21/17

        DHP
        sorry...I did mistype...I have 21/18 gears..and wish to install 21/17

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        • #5
          Re: Changing omc upper gear ratio?

          Sorry about the mis type...I have 21/18 gears..and wish to install 21/17

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          • #6
            Re: Changing omc upper gear ratio?

            bump

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            • #7
              Re: Changing omc upper gear ratio?

              the ratios are so close I think you'll be fine - might require a prop change of 2" pitch.
              What pitch of prop are you running now? If 19 or 21P, then you have room either side to change pitch.

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              • #8
                Re: Changing omc upper gear ratio?

                The difference will be about 5% at your output shaft. 21/18 = 1.167, 21/17 = 1.236. Not real familiar with OMC's don't know which side is driven and which side is driving. If the 21 is the driving gear going from the 18 to the 17 will be the equivelent of going to a prop about an inch bigger in pitch. Your WOT RPM should drop 100 - 200 RPM and you'll have a little less holeshot.

                If the small gear is the driving gear it'll work out the opposite way.

                As kjdkjd said, it'll probably be so close you wouldn't notice.
                Any opinions expressed above are worth exactly what you paid for them!

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                • #9
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                  Re: Changing omc upper gear ratio?

                  Thanks for the replies. I will go with the 21/17...since its free...I cant beat it.

                  Jim

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