Leaking Hypoid oil... from the seam between tire and final drive

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  • WilliamGLX81
    • Feb 2026
    #1

    Leaking Hypoid oil... from the seam between tire and final drive

    I recently changed my final drive oil (The 90 turn at the rear of the bike. It's what would be a differential if I had 2 wheels.)

    Then, I noticed that I was accumulating oil on my recently cleaned wheels >.<
    I checked the fill and drain bolts, both of which were tight and sealed. Is it the breather box in the final drive that may be leaking? Where do I start? I assume by dropping the rear swing arm and disassembling the swing arm. What am I looking for at that point?

    I also developed what sounded like a microphone set to reverb. It only happens when I'm already running at 3-4K rpm (need a tach cable, just got my new gauge set) and I push it harder. It does accelerate, but the sound is quite frightening.
    I opened the final drive and put my pinky into the fill hole. I couldn't feel any fluid. I added a few ounces until I could feel it inside and some came out the fill hole. I'll test drive it and see if it's fixed the noise.
    I've currently got a car with a blown head gasket, a newly acquired car that needs a MAF sensor and motor/transmission mounts.
    I re-he-heaaally don't want a broken down motorcycle to boot.
    Tags: None
    • BassCliff
      #2
      Hi,

      Have you replaced the crush washers on the drain hole and fill hole bolts? Don't overfill or it will leak too.


      Thank you for your indulgence,

      BassCliff

        Comment

        • SqDancerLynn1
          #3
          There is a seal & O ring shouldn't be too hard to replace $30 for parts. Make sure it is the seal and not an over greased wheel adapter spline

            Comment

            • WilliamGLX81
              #4
              Thanks for both replies.

              Is the idea simply to fill to a level equal to the fill-hole?

              I can double check the washers, but the leak is definitely from between the wheel and the final drive unit (Am I calling it the right part? :/ )
              I thought it might have been my over-greasing of the drive splines in the wheel gear, but I waited for the dripping grease and checked it - it's definitely the 90w gear oil, not the bearing grease (I figure bearing grease is like... 300wt).

              I remember the manual mentioning a breather box in the hypoid/final drive unit. Could that be the culprit?

              Any idea if an under-lubed final drive will make a shuddering noise/harsh vibe under heavy load? My gut instincts say yes. I'll test drive it.... probably friday.

                Comment

                • Larry D
                  Forum Sage
                  Past Site Supporter
                  • Jul 2008
                  • 4926
                  • Indianapolis,IN
                  #5
                  From Cliff's site.

                  http://members.dslextreme.com/users/bikecliff/images/reardiff.pdf
                  Larry D
                  1980 GS450S
                  1981 GS450S
                  2003 Heritage Softtail

                    Comment

                    • WilliamGLX81
                      #6
                      AHHHH! Sounds like I may have been actually squirting oil into the breather system....
                      I had to drop my exhaust and my fill hole is parallel to the floor. I was squeezing the bottle and squirting the oil in the hole. Maybe I filled up the breather system. I'll have to deal with that when I get back.

                      Thanks a bunch

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