Alternative diff in a Cooper S!


  1. Hi there,

    As you'll probably know from previous postings, I am building a Mk1 1275 S replica at the moment.

    Has anyone had any experience in fitting a 3.1:1 diff in a 1275 S ?

    I know the speedo will be wrong but I can get that recalibrated.As most of my journeys will be medium to long distance I was thinking about the 3.1 diff to reduce the engine revs at cruising speed but am not sure whether the trade off with the acceleration is too much.

    The engine is astandard 1275 S engine bored to +60.

    Any views anyone???


  2. imported post

    A 3.1 will be no problem in a 1275 S, and if you use a 7 tooth speedo drive gear in place of the original 6 tooth item the speedo is not normally too far out either.

    On a longtrip the car will be much more pleasant at higher speeds, and as a further bonus will be more economical

    The only other modification required to fit this is to the mainshaft double bearing cap retainer, which will need grinding out, (or milling), to clear the bigger pinion, (I normally use a die grinder with a tungsten carbide cutter to achieve this).

    Hope this helps you.

  3. imported post

    Hi 64CooperS,

    Thanks for your comments on this and the very useful and essential tips as well. Its a great help 😀

    I only wished locating a pair of Hardy Spicer output flanges and diff side covers were as easy as this one:(

    Kind regards


  4. imported post

    Hi 64CooperS

    Will I need to change the speedo drive as well as the pinion? Can I use the complete drive gear from a gearbox that I take the 3.1 diff out of. I've got a complete mid 80's A+ engine/box that I am shouldhave the 3.1 diff? I've been reading Keith Calvers write up on the Mini Spares Website and althout very informative, it has confused me a bit – not difficult !!!



  5. imported post

    Use the 80's 7 tooth metal gear with your original nylon speedo gear.

    With regards the driveshafts unless you want a totally original specification you can also convert to pot joints using the parts from your 80's gearbox.

  6. imported post

    Many thanks for clarifying that 64CooperS.

    Iwant to maintain an original specification where possible andhave managed to obtain a further complete Hardy Spicer driveshaft components, including some flanges and covers since I posted my last comment. Didn't really want a complete set but couldn't find the seperate bits I wanted. Bet they'll turn up now!!



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