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Differential Rebuild Recommendations please

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    #16
    Originally posted by Robmsharp View Post
    In preparing to do this job I've read the ROM and it seems to be a straightforward job of removing the Hypoid, am I right to assume the propshaft and nose extension piece stay in situ and the Hypoid pulls off the splined shaft?
    I'm also planning to fit the 3:45 ratio unit as many people have recommended but in doing this is there anything extra I need to consider before doing the job and are there any 'gotchas' anyone can advise upon?
    I was about to put my new Fitchett one in but thought I would check nose extension housing first and change the bearing, especially after watching richardthestag video on YouTube (https://www.youtube.com/watch?v=U59lS5Hmfys&t=319s). Glad I did as mine was cracked in the usual place. I got a nice reinforced and powder coated one from EJ Wards with a new RHP bearing.

    Best to check it as failure is, I have read, catastrophic at speed.

    Jeff
    Last edited by JeffW; 18 February 2021, 10:10.

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      #17
      Interesting video. Nasty crack developing there and looks like it was caught just in time, not far from disasterous failure.

      As for the diff itself, that has been running like that for some considerable time I suspect for the bronze shim to completely disappear.
      Not sure I would describe it as catastrophic though, it is relatively easy to examine closely and cheap to fix, assuming the planet gears
      and bearings are all ok & it was quiet in operation.
      You don't even need to remove the crownwheel assembly from the casing to do it.

      I have a spare diff nose extension, I think I might do a bit of welding, and more regular inspection of the one on the car.

      Neil.
      Last edited by NeilR; 18 February 2021, 13:15.

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        #18
        Originally posted by NeilR View Post
        Interesting video. Nasty crack developing there and looks like it was caught just in time, not far from disasterous failure.

        As for the diff itself, that has been running like that for some considerable time I suspect for the bronze shim to completely disappear.
        Not sure I would describe it as catastrophic though, it is relatively easy to examine closely and cheap to fix, assuming the planet gears
        and bearings are all ok & it was quiet in operation.
        You don't even need to remove the crownwheel assembly from the casing to do it.

        I have a spare diff nose extension, I think I might do a bit of welding, and more regular inspection of the one on the car.

        Neil.
        Hi Neil

        Sorry for the confusion, I meant the failure of the nose extension where the crack was can have catastrophic consequences. It happened to member 'staggering' a while back https://www.socforum.com/forum/forum...9090-diff-nose (post #2) and he was very lucky it was not far worse.

        Jeff

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          #19
          I have a diff in parts. not easy to get some sizes of planetary gear shims. I have another thread on a particular size shim I'm looking for.

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            #20
            No problem Jeff.

            Neil

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              #21
              Thanks for your replies and advice guys.
              I love deadlines - I like the whooshing sound they make as they pass by!

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                #22
                When I changed mine (J man od) to a 3.45 I fitted a speedo drive gear from a 2.5 S to correct the ratio change. Speedo is spot on against TomTom at 100k. I don't know if the same drive gear is used in a BW35.
                Cheers Ian A

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                  #23
                  I was wondering how far the speedo would be out with a 3:45 diff, can anyone recommend where the best place to buy the correct speedo drive gear from?
                  I love deadlines - I like the whooshing sound they make as they pass by!

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                    #24
                    Originally posted by Robmsharp View Post
                    I was wondering how far the speedo would be out with a 3:45 diff, can anyone recommend where the best place to buy the correct speedo drive gear from?
                    3.45:1 diff will give a theoretical error of approx 7%. This can easily be cancelled out by errors within the speedometer itself, or by fitment of undersized tyres. It may be worth waiting to see how accurate your speedometer is after fitting the diff, by using a GPS speedometer app.
                    Dave
                    1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

                    Comment


                      #25
                      Originally posted by DJT View Post

                      3.45:1 diff will give a theoretical error of approx 7%. This can easily be cancelled out by errors within the speedometer itself, or by fitment of undersized tyres. It may be worth waiting to see how accurate your speedometer is after fitting the diff, by using a GPS speedometer app.
                      Surely by fitting undersize tyres you will defeat the object of changing the diff ratio??
                      Auto transmission rebuilding since 1979 - for my sins!

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                        #26
                        Originally posted by Alacrity View Post

                        Surely by fitting undersize tyres you will defeat the object of changing the diff ratio??
                        Er, yes. I wasn’t recommending, just pointing out that IF undersized tyres are fitted, it could cancel out the speedometer error caused by fitting the higher ratio diff.
                        Dave
                        1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

                        Comment


                          #27
                          I fitted a speedo from a 2500 S SN6215/12 - 840 when I had a ZF installed, it gives me reasonable good accuracy against my sat nav, I also have 15" wheels with 195/65 tyres.

                          Attached is some info from Chris Witors site relating to Speedos for 2500 models, I would assume that as my speedo indicates 840 as shown under "Speedo Ratio revs/mile" for 3:45 ratio that this would be the pinion that could give a more accurate reading than the original one.

                          Nigel
                          1976 MK2 TV8 ZF-4 HP-22 Auto, Inca Yellow, Minilite Replicas, Stagdad Pump

                          Comment


                            #28
                            I think Stag speedos are normally 1000 tpm..

                            You just need to find the tpm (turns per mile) of your tyre, (many websites) times diff ratio divided by speedo drive ratio (its 3:1 for manuals, probably same for auto) and see what number comes up, the nearer to 1000 (or 850 for saloon speedo) the more accurate it will be.

                            The 3.4

                            195/70-14 =814tpmx3.45/3=936
                            185/80-14=786x3.45/3=903

                            just for grins the 3.7

                            195/70-14=815x3.7/3=1005
                            185/80-14= 786x3.7/3=969

                            I'm guessing that if you count the number of teeth on the original drive, divide it by 3 you will know the no of teeth on the drive gear, from that you can work out the teeth needed to get the speedo drive ratio to make it it bang on.

                            OR someone will tell you


                            Terry, yes Bored again..
                            Last edited by trunt; 24 February 2021, 20:22.
                            Terry Hunt, Wilmington Delaware

                            www.terryhunt.co.uk

                            Comment


                              #29
                              Its been bugging me why the 185/80-14 tyres come out lower than 1000 and would under-read.. So I read up and the tpm calculators show unloaded and apparently you should add 3% for loaded, i.e. tyres slightly flattened by the weight of the car.

                              185/80-14 840 (815+3%) x3.7 /3 = 1036

                              That's better!
                              Terry Hunt, Wilmington Delaware

                              www.terryhunt.co.uk

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