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 Rear axle diff
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bigt80

United Kingdom
944 Posts

Posted - 07 Jan 2018 :  16:22:59  Show Profile  Reply with Quote
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
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pruyter

Netherlands
203 Posts

Posted - 31 Dec 2017 :  13:48:57  Show Profile  Reply with Quote
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.

Regards,

Peter
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pruyter

Netherlands
203 Posts

Posted - 30 Dec 2017 :  11:30:37  Show Profile  Reply with Quote
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.

Regards,

Peter
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Sunbeam Tommy

United Kingdom
7 Posts

Posted - 29 Dec 2017 :  13:14:07  Show Profile  Reply with Quote
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?
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pruyter

Netherlands
203 Posts

Posted - 29 Dec 2017 :  10:46:37  Show Profile  Reply with Quote
Yes did. Regarding my Rapier series IV, fitted with overdrive, I changed the diff from 4.22 to 3.89.

Regards,

Peter
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Paul_White

United Kingdom
41 Posts

Posted - 28 Dec 2017 :  23:29:05  Show Profile  Visit Paul_White's Homepage  Reply with Quote
Tallest ratio is 3.7:1 what do you have now?
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Sunbeam Tommy

United Kingdom
7 Posts

Posted - 28 Dec 2017 :  13:30:05  Show Profile  Reply with Quote
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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