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Sunbeam Tommy Posted - 28 Dec 2017 : 13:30:05
Hi there,
Has anyone fitted a different ratio rear diff to the rear axle of a Rapier to lower engine revs on the motorway? Just wondered.
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bigt80 Posted - 07 Jan 2018 : 16:22:59
The crown wheel to pinion teeth is also stamped on the crown wheel edge on the arrow range if I remember correctly?
Great info there Peter :-). G
pruyter Posted - 31 Dec 2017 : 13:48:57
I forgot to mention how you indentify a diff when it is not installed in a car. It works by counting the teeth of both the crown wheel and the pignon.

3.89: crown wheel 35 teeth and pignon 9 teeth;
4.22: crown wheel 38 teeth and pignon 9 teeth;
3.70: crown wheel 37 teeth and pignon 10 teeth.


pruyter Posted - 30 Dec 2017 : 11:30:37
You can use diffs from both Audax and Arrow types of Rootes cars. On my Rapier series IV I have installed a diff from a Rapier H120 without any problem except that you have to be aware of the fact that there is difference in lengt of the nose of the diff. There are two ways to counter this problem: no 1 shortening the propellor shaft if you go from Arrow to Audax and no 2 install the nose from the Audax on the diff from the Arrow. I went for the last option.

To check which differential you have, jack up one rear wheel (clock the other one well), put the car in top gear and use the starting handle to bring the engine to top-dead-centre(watch the timing marks). Now mark the bottom centre of the tyre whilst holding the wheel back against transmission backlash. Turn the engine over two complete turns to exactly TDC again and,holding the wheel back agains backlash once more, see how far the wheel has rotated.
If you have a 3.89 diff, the wheel should have done just over one complete turn and if you have a 4.22 diff it should have done less than one turn. The exact rotations are:
4.22 gives 341 degrees;
3.89 gives 370 degrees;
3.70 gives 389 degrees.


Sunbeam Tommy Posted - 29 Dec 2017 : 13:14:07
I am not sure what my diff is but it seems low-geared. Where do I look for the info? Also, if it is a low one, where would I source another? Presumably the diffs are common to other Rootes vehicles?
pruyter Posted - 29 Dec 2017 : 10:46:37
Yes did. Regarding my Rapier series IV, fitted with overdrive, I changed the diff from 4.22 to 3.89.


Paul_White Posted - 28 Dec 2017 : 23:29:05
Tallest ratio is 3.7:1 what do you have now?

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